Breakthrough in Quantum Key Distribution: Chinese Scientists Develop On-Demand Single-Photon Source for Unbreakable Encryption

Quantum key distribution (QKD), an encryption method grounded in the principles of quantum physics, is widely regarded as one of the most promising techniques for securing digital communications. It enables the transmission of encryption keys via the quantum states of photons—states that cannot be copied or measured without disruption—thus rendering interception virtually impossible without detection.

Due to the technical challenges of generating true single-photon sources, most current QKD systems rely on attenuated laser pulses that simulate single photons. However, these pulses often fail to contain the desired number of photons, and only about a third are actually suitable for generating secure keys.

Researchers at the University of Science and Technology of China have now overcome this limitation by deploying a source capable of emitting individual photons on demand. Their newly developed system, described in Physical Review Letters, achieves a significantly higher rate of secure key generation than previous methods.

According to co-author Feihu Xu, QKD protocols have traditionally employed weak coherent pulses and decoy-state protocols. This approach, however, is constrained by a theoretical limitation—the probability of single-photon emission cannot exceed approximately 37 percent, which hampers overall efficiency. Although single-photon sources could theoretically bypass this constraint, their historically low brightness has hindered practical demonstration—a challenge that had remained unresolved for nearly two decades.

The primary goal of the new study was to engineer a physical system capable of emitting high-brightness single photons, thereby overcoming the deficiencies of conventional laser-based sources and enhancing the reliability of QKD technology.

To achieve this, the team utilized quantum dots, resonant microcavities, narrow-band filtering, and efficient polarization modulation, culminating in what is currently the most effective single-photon source for QKD. Experiments conducted both in laboratory settings and under real-world conditions affirmed the system’s superior performance and marked increase in key generation speed.

During trials, the new system surpassed the fundamental rate limits associated with conventional coherent pulses. In a field experiment with signal losses approaching 15 decibels, the key generation rate exceeded previous benchmarks by 79 percent. Nonetheless, Xu noted that the system’s current tolerance for channel loss remains somewhat lower than that of traditional methods.

This limitation is attributed to residual multi-photon emission effects observed when employing non-decoy-state protocols. Moving forward, the researchers aim to bolster the system’s loss tolerance by refining the photon source and integrating advanced protocols.

The team also intends to enhance photon purity and emission efficiency, further advance QKD system performance, and explore the use of protocols such as the decoy-state method. Additionally, their roadmap includes the development of a more intricate quantum networking infrastructure, encompassing teleportation, quantum relays, and repeaters. The scientists are confident that continued technological progress will eventually render quantum key distribution viable for everyday use.