Maintaining long-term data and communications security is essential. The development of quantum computers has sparked a major security dilemma regarding conventional network system [1]. In response to this situation quantum cryptosystems have been emerged, C. H. Bennett and G. Brassard proposed the first quantum cryptography protocol to guarantee the security of information unconditionally based on the laws of quantum physics in 1984 [2]. The BB84 protocol uses the four phase-time or polarization states of a single photon to encode random key information. The random keys are generated and then encoded onto a quantum state for transmission at Alice, the keys are encoded at Bob to complete the distribution. Its security is judged by the bit error rate (BER) of the data retained by Alice and Bob after base comparison. Many current quantum key distribution (QKD) systems are implemented using weakly coherent pulses, which makes them susceptible to photon number splitting (PNS) attacks on the multi-photon pulses in them [3,4]. Therefore, Hwang introduced the first decoy state protocol to detect PNS attacks and Wang and Lo improved the decoy state protocol by proposing two-decoy protocol and multi-decoy protocol [5,6]. The simpler method with only a single and a decoy state protocol was presented, and D. Rusca took into account finite size effects to demonstrate that the one-decoy state is sufficient to approach or achieve the system performance of the two-decoy method in 2018 [7].

Bulk optics experiments are demonstrated in QKD [[8], [9], [10]], but it's complex and less flexible. Recent efforts have realized systems for QKD using various integration platforms allowing for high performance elements of increasing complexity, such as silicon-on-insulator (SOI) [[11], [12], [13]], lithium niobate (LN) [14,15] and silica-on-silicon planar lightwave circuits (PLCs) [16,17]. The silica-on-silicon PLCs lead to excellent mode-matching with standard optical fibers and minimal loss, suitable as a decoder of QKD systems. In the last few decades, researchers have taken attention to the PLC platform and used PLCs to fulfill QKD experiments using BB84 time-bin protocol. In 2010, A. Tomita generated sift key at the rate of 2.4 kbps based PLC through a 97-km single installed fiber core using BB84 time-bin protocol at a clock repetition rate of 625 MHz [18]. In 2022, a transmitter chip based on hybrid-integration of silica PLC and LN modulator encoded four BB84 time-bin states, and an asymptotic secure key rate is 10.041 kbps over 20-km at a clock repetition rate of 156.25 MHz [14], but the key rate has not been obtained through experiment. In the same year, a polarization-insensitive time-bin decoder chip with the hybrid asymmetric Faraday-Michelson interferometer (AFMI) based on PLC was developed and an average secure key rate is 1.34 Mbps over a 50 km fiber channel at a clock repetition rate of 1.25 GHz [19]. However, the first two systems did not apply experimentally a decoy state method and the chip are all incompletely packaged in these researches.

In order to further enhance performance, we demonstrate a simplified BB84 time-bin QKD experiment system with one-decoy state method that encodes using time basis(Z) and estimates the error rate using phase basis(X). Since the generation of the time basis does not pass through the decoding end, it significantly reduces the losses at the decoding end, specifically only including the losses caused by optical splitters. The key rate is negatively correlated with the losses at the decoding end [18], thereby greatly increasing the secure key rate. In addition, two PLC asymmetric Mach-Zehnder interferometer (AMZI) modules integrated an accuracy of 0.01 °C temperature control system are employed to enhance integration, system flexibility, and practicality. And we achieve both transmit and receive functions. According to our analytical model, when the system repetition frequency is up to 1.25 GHz, the estimated key rate is 3.4 Mbps over a 20 km SMF. Limited by the experimental equipment, a secure key rate of about 531 kbps is measured in the experiment when a repetition frequency of 156.25 MHz is used, greatly exceeding the current level at the same repetition frequency.