# Low-Frequency Communication Based on Rydberg-Atom Receiver
## 1 Introduction  
Low-frequency fields below 300 kHz [1] offer numerous advantages in communication, including low loss, high bypass capability, long range, and low absorption loss in seawater and soil [2]. However, the traditional low-frequency receiver with a large-scale metal antenna is restricted from further application. Rydberg atoms provide an alternative approach in low-frequency communication, featuring compact sizes of the order of centimeters and decoupling from electric field frequency [3]. Rydberg atoms exist in a high energy state, characterized by their calibration-free SI traceability, large polarizabilities, and transition dipole moments [4]. These atoms are highly excited, with one or more electrons occupying states defined by large principal quantum numbers (n \(>\) 30) [5]. They exhibit significantly larger average radii and enhanced sensitivity to external electric and magnetic fields, making them ideal for quantum sensing and communication applications. The polarizability and dipole moment of Rydberg atoms scale with the principal quantum number as n\({}^{7}\) and n\({}^{2}\)[6], respectively. A Rydberg atom sensor is capable of sensitively detecting electric field signals from DC to THz [5]. Utilizing electromagnetically induced transparency-Autler Townes (EIT-AT) effect or Stark shift effect [7, 8] enables the measurement of factors such as the amplitude, phase, and polarization of the electric field [9, 10]. Researchers have focusedon developing Rydberg-atom-based receivers capable of operating across a wide frequency range, from 300 MHz to 300 GHz [11; 12; 13; 14; 15; 16; 17]. Additionally, Rydberg-atom-based receivers have successfully demonstrated the reception of video and image signals [18; 19].  
Current communication technologies based on Rydberg atoms predominantly operate in the high-frequency band, with relatively few efforts focused on exploring the low-frequency spectrum. The shielding effect of the alkali-metal-atom container on low-frequency electric fields [20] makes it challenging to detect signals below the MHz range. Such obstacles hinder the development of low-frequency communication receivers based on Rydberg atoms. There are two approaches to address this limitation. The first approach involves constructing the vapor cell using sapphire [21; 22], which exhibits significantly higher internal surface resistance compared to glass. However, this method incurs a significantly higher cost. The second approach involves integrating two parallel electrode plates inside the vapor cell, allowing low-frequency signals to be directly introduced into the cell via these plates [23; 24]. While cost-effective, this methodology may compromise the hermetic seal of the vapor cell.  
In this study, we demonstrate low-frequency communication using Rydberg atoms in a vapor cell equipped with two parallel electrode plates. All components of the receiver front-end are passive devices, and the signal detection method is entirely optical. We first present the spectrum of the signal received by Rydberg atoms under three modulation modes: OOK, BPSK, and 2 FSK. The SNR of the received signal is analyzed as a function of various emission voltages. Subsequently, we evaluate different parameters at different symbol rates to assess the quality of Rydberg-atom-based low-frequency communication, including IQ constellation diagram, EVM, and eye diagram. We also demonstrate high-fidelity digital image transmission based on the low-frequency communication based on Rydberg atoms, yielding a PSNR of 70 dB for image transmission fidelity. This study demonstrates the significant potential of Rydberg atoms in the field of low-frequency communication.