With the rapid advancement of network technology and the widespread adoption of smart mobile devices, while technology brings convenience, it also brings unprecedented cybersecurity threats and challenges. Against this backdrop, the demand for information security is increasing daily, and the technological requirements for protecting information security in next-generation communications are constantly rising. Quantum communication technology, with its unique physical characteristics and security advantages, has become a key technology for next-generation information security and is expected to become an important cornerstone for safeguarding digital communication security.
Professor Hao-Chung Kuo, Director of the Semiconductor Laboratory at Hon Hai Research Institute (HHRI) and Chair Professor at National Chiao Tung University (NCTU), along with his research team, collaborated with research teams from NCTU and NTU, and the National Institute of Information and Communications Technology (NICT) of Japan. They successfully improved the Differential Phase Shift Quantum Key Distribution (DPS-QKD) technology, proposing a new asynchronous bit rate encoding/decoding scheme that enhances the stability and reduces the error rate of quantum key distribution. This innovative achievement was published in the international journal *APL Photonics* and has received significant attention. Details of the paper can be found at: https://pubs.aip.org/aip/app/article/9/12/126115/3328370/Asynchronous-bit-rate-differential-phase-shift
Figure 1. Architecture diagram of a DPS-QKD system. It includes a generator (AWG and amplifier), drivers (current source, power supply, and bias T-connector), a transmitter (DFBLD), modulators (intensity and phase modulators), fiber optic components (single-mode fiber winding and attenuators), a decoder (delay interferometer), and a receiver (SPAD, DC blocker, and DSA). The illustration below shows three types of DPS-QKD encoding, decoding, and detection schemes: synchronous (left), asynchronous (middle), and an enhanced asynchronous scheme incorporating decoy states (right), for comparison.
This study utilizes asynchronous encoding/decoding technology to extend the free spectral range (FSR) of the delay line interferometer (DLI), successfully improving the tolerance of the DPS-QKD system to environmental thermal disturbances. The research team tested four fiber-based DLIs with different FSRs, ranging from 40 MHz to 1 GHz. The results showed that extending the FSR to 1 GHz reduced the qubit error rate (QBER) to 2.2% and increased the secure key rate (SKR) to 77.32 kbps.
In addition, the team selected a distributed feedback laser diode (DFBLD) with a linewidth of only 296.66 kHz and excellent wavelength stability as the light source, successfully controlling the wavelength perturbation within ±0.05 pm and significantly reducing long-term decoding errors.
Figure 2. Decoding performance of asynchronous bit rate DPS-QKD streams using different fiberized delay interferometers (DLI) with corresponding free spectral ranges (FSR) of 40, 192 MHz, and 1 GHz: (a) qubit error rate (QBER) and secure key rate (SKR) obtained at DLI visibility of 91.76%; (b) QBER and SKR obtained at the maximum DLI visibility (approximately 96%); (c) simulated relationship between visibility and temperature fluctuations; and (d) magnified view of the slope of visibility versus temperature gradient, which is suppressed with increasing FSR.
Compared to traditional synchronous technologies, asynchronous DPS-QKD technology offers significant advantages in improving immunity to thermal disturbances, while effectively reducing reliance on high-precision temperature and current control equipment, further lowering system operating costs. The short-path DLI design further enhances interference stability, enabling the system to operate stably for several minutes or more under varying environmental conditions.
The study indicates that this technology also possesses flexibility, allowing for dynamic adjustments between high encoding and low decoding rates to achieve low-power and highly secure quantum key distribution. This technological breakthrough paves the way for the application of quantum key distribution in cybersecurity, finance, and the military.
This technology not only demonstrates a revolutionary advancement in the security and stability of quantum communication systems, but also lays a solid foundation for the future commercial application of quantum encryption technology. The research team emphasizes that this breakthrough will be a significant milestone in promoting the development of global quantum communication. In the future, they plan to develop more efficient decoding algorithms to further realize the deployment of large-scale quantum communication systems and promote the realization of a new generation of secure communication networks.
