## 1 Introduction  
Quantum key distribution (QKD) [1; 2; 3; 4] allows legal participants to share a secret key and enables theoretically information-secure communications [5; 6; 7]. Recently, it has spurred much interest, resulting in two approaches, i.e., discrete-variable (DV) QKD [8; 9] and continuous-variable (CV) QKD [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. A possible breakthrough may come from the practical implementation of CVQKD, which has an advantage of being practically implemented with modern telecommunications, thus allowing for the exploitation of the heritage of integrability in existing optical communication system. At present, there are several mainstream modulations in CVQKD, such as Gaussian modulation (GM) [11; 12; 13; 14; 15] and discrete modulation (DM) [16; 17; 18; 19; 20]. In the former, the transmitter generates the key bits in the quadratures of the Gaussian modulated optical field, while the receiver recovers the secret key using a coherent detector [13]. It is GM-CVQKD that could achieve the potential high secret key rate. Unfortunately, it is still limited to the transmission distance, compared with its DVQKD counterpart. A challenge is that the resulting reconciliation efficiency [14; 15] is still low, which is unsuitable for long-distance transmission, whereas in the latter, one can solve this problem by using a DM source for CVQKD [16], where it generates nonorthogonal coherent states, taking advantage of the randomly measured quadrature for information encoding rather than exploiting the fixed quadrature in source preparation [17]. Consequently, it results in a merit of both high reconciliation efficiency and high tolerance against the excess noise [11; 12; 13; 14; 15] such that it can extend the transmission distance of the CVQKDsystem [18]. Actually, GM-CVQKD is simpler for theoretical performance analysis than DM-CVQKD, and the secret key rates can approach the theoretical limits [17], whereas GM-CVQKD itself is alway approximated by the DM sources, which may be taken into account for security analysis [18]. Moreover, as GM-CVQKD usually requires more resources in terms of randomness and classical post-processing resources, DM-CVQKD can offer further simplified implementation in experiments. While theories and experiments of DM-CVQKD have suggested that we can achieve the above-mentioned goal, there are still some potential challenges while promoting its performances. Currently, photon catalysis (PC) [21; 22; 23; 24; 25], which is a non-Gaussian operation with \(n\)-photon state input/output in essence, has been utilized for security improvement attributed to the photon catalyzing operation, which increases the entanglement degree of the two-participant system by increasing the correlation between the transmitter and the receiver [21]. In a certain low-squeezing parameter, the entanglement of the two-mode squeezed vacuum (TMSV) state can be enhanced by increasing the catalysis photon number [22]. Due to equivalence of the entanglement-based (EB) CVQKD system and the prepare-and-measure (PM) CVQKD system [11; 12; 13; 14; 15; 16; 17; 18; 19; 20], the PC-involved DM-CVQKD system may be implemented with existing optical technologies. Motivated by the above advantages, we propose an improved DM-CVQKD system through photon catalyzing. The DM scheme is employed for the source preparation because it can tolerate a lower signal-to-noise ratio (SNR) well, resulting in the long-distance transmission of the CVQKD system. Meanwhile, a zero-photon catalysis (ZPC) operation with zero-photon state input/output is adopted in the DM-involved source preparation at the transmitter. The ZPC-based CVQKD protocol has been proven to be beneficial for tolerating lower reconciliation efficiency [24; 25] and extending the maximal transmission distance, thus promoting its practical implementations with underlying technology.