Simultaneous transmission of information and key exchange using the same photonic quantum states

Quantum communication realizes information-theoretic security using photonic quantum states, for example, quantum secure direct communication (QSDC), which can achieve secure and reliable communication in a channel with both noise and eavesdroppers. However, QSDC suffers from large losses and short communication distances, thus being impractical for applications. Here, we have proposed a one-way quasi-QSDC protocol with single photons. This protocol enables the simultaneous transmission of information and key exchange using the same single photons and is robust against loss and error because it uses error correction and spectrum expansion techniques. In a proof-of-principle demonstration using weak coherent pulses, the system achieved a real-time secure transmission rate of 2.38 kilobits per second over a 104.8-km standard telecommunication fiber, which set world records in both aspects. This system paved the way for the practical application of QSDC and offers a unique method to detect eavesdropping online, which is crucial in certain circumstances.

Methods
To demonstrate the feasibility of the proposed STIKE protocol, we completed a proof-of-principle demonstration experiment. We constructed a Faraday-Michelson system to run the STIKE protocol. The communication distance between Alice and Bob was 104.8 km, utilizing standard telecommunication fiber that exhibited a loss of 0.2 dB/km. 

We conducted a 168-hour test of the communication rate and quantum bit error rate (QBER) of the system, with the data summarized in Fig2.xlsx. The communication rate and QBER data are also separately available in Rate_168h.txt and QBER_168h.txt, respectively, facilitating the plotting of temporal trends in both metrics using the Matlab script Fig.m, corresponding to Figure 2 in the paper. Additionally, Matlab scripts Fig3.m and h.m were utilized for the performance analysis of the secrecy capacity in the STIKE protocol, resulting in the generation of Figure 3. This figure encompasses our experimental communication rate results alongside simulations of the secrecy capacity and reliable communication bound of the protocol using the two-decoy state method and wire-tap channel theory.