# Atomic Receiver by Utilizing Multiple Radio-Frequency Coupling at Rydberg States of Rubidium
## 1 Introduction  
With the new developments in precision measurement technology, numerous atom-based sensors and communication devices have been proposed and verified in laboratories. Many new concepts and technologies of Radio Frequency Electric field (RF E-field) measurements in the quantum sensing domain have been presented, such as Rydberg atoms. Because of their high precision, sensitivity, and as the measurement results can be directly traced to the international system of units (SI) via atomic constant [1, 2], they have great potential in many application scenarios. Rydberg atoms are some kinds of atoms with large principal quantum numbers [3]. Their large dc polarizabilities and microwave-transition dipole moments [4], Ref. [5] make them very suitable for the RF E-field measurement. Combining the electromagnetically induced transparency (EIT) [6, 7] Autler-Townes (AT) [8] effects, measurement of the RF E-field with Rydberg atoms can be converted into frequency measurement. The sensitivity of the RF E-field has been observed to be less than 1 \(\mu\)V\(\cdot\)cm\({}^{-1}\).Hz\({}^{-1/2}\)[9] under the predictable noise limit, and the measurement uncertainty has been observed to be less than 0.5% [2]. Rydberg atoms are also extensively employed in many fields, like high-resolution imaging, millimeter-wave/terahertz [10], and antenna measurement [11]. Owing to the high sensitivity of Rydberg atoms to RF E-field, the concept of wireless communication using Rydberg atoms has been recently proposed and verified. [PERSON] et al. [12] proposed a new Rydberg atom-based wireless fiber-optic signal transmission system, which utilized the Rydberg atoms as a receiving antenna, and the signals were directly recovered at the baseband without any demodulation means. [PERSON] et al. [13] used Rydberg atoms as an optical antenna to realize the 8-state phase-shift-keying (PSK) digital communication protocol. [PERSON] et al. [14] demonstrated an atomic receiver for amplitude modulation (AM) and frequency modulation (FM) communication with a 3 dB bandwidth in the baseband of 100 kHz. [PERSON] et al. [15; 16] investigated the feasibility of this atom-based receiver over a continuously tunable RF-carrier; the authors showed that the communication at a rate of 500 kbps can be reliably performed within a tunable bandwidth of 200 MHz near a 10.22 GHz carrier. [PERSON] et al. [17] observed that the receiving system based on Rydberg atoms can detect and demodulate binary-phase-shift-keying (BPSK), quadrature-amplitude-modulation (QAM), and quadrature-phase-shift-keying (QPSK) signals. Due to numerous available resonant frequencies of the Rydberg states from 100 MHz to sub-THz, it is possible to achieve the broadband communication by changing the coupling frequency between RF E-field and the Rydberg States. Therefore, Rydberg atoms are good candidates for microwave communication applications. All the aforementioned communication experiments transmitted data over a single RF-carrier at atomic resonance frequency. As the development of wireless communications, spectrum resource is increasingly scarce, and interference between systems has become increasingly serious. It is of great necessity to find an optimal way to make better use of the limited spectrum resource. Using the abundant microwave-transition energy levels between Rydberg states, instead of recovering the time-varying signal on the specific RF-carrier frequency, we simultaneously couple resonant and detuning microwaves to verify the feasibility of dual-carrier communication and achieve frequency division multiplexing (FDM). This can improve spectral performance and allow real-time channel multiplexing. In this paper, the feasibility of digital FDM communication is proposed, together with the atomic receiver by using multiple RF couplings. Additionally, we analyze the change of signal-to-noise ratio (SNR) by scanning the coupling laser frequency. The feasible experimental conditions which are selected by the SNR judgment are discussed.