5-GHz chip-based quantum key distribution with 1Mbps secure key rate over 150 km

TL;DR

The paper tackles the challenge of achieving high secure key rates (SKR) for QKD over intercity distances by implementing a fully integrated $5$-GHz QKD platform that combines a high-speed laser, ultra-low-jitter detectors, and advanced polarization-state preparation. The authors introduce a dual-MZI polarization-state preparation method that enables two near-perfect mutually unbiased bases (MUBs) even with relatively modest 2D-GC polarization isolation, along with a twin-layer SNSPD design that delivers high detection efficiency and ultralow timing jitter. Experimentally, the system achieves an SKR of $1.076$ Mbps at $150$ km and $105$ kbps at $200$ km over standard fiber, with QBERs around $0.47$–$0.51$ ext{%.} The work also demonstrates strong 6-hour stability (average SKR $1.025 ext{ Mbps}$) and projects substantial potential improvements (e.g., up to $550$ Mbps at $5$ km and $228$ Mbps at $25$ km), highlighting the practicality of high-SKR BB84 QKD for mid-range backbone networks and intercity links.

Abstract

Quantum key distribution (QKD) enables secure communication by harnessing the fundamental principles of quantum physics, which inherently guarantee information-theoretic security and intrinsic resistance to quantum computing attacks. However, the secure key rate of QKD typically decreases exponentially with increasing channel distance. In this work, by developing a novel polarization-state preparation method, an ultra-low time-jitter laser source and superconducting nanowire single-photon detectors, we demonstrate a 5-GHz integrated QKD system featuring ultra-low quantum bit error rates (QBERs). The system achieves secure key rates of 1.076 Mbps at 150 km and 105 kbps at 200 km over standard single-mode fiber channels, respectively. Our system substantially enhances the secure key rate, enabling high-resolution video calls with one-time-pad encryption over intercity backbone QKD links. This work represents a significant step forward in the development of high-performance practical QKD systems.

Figures (4)

• Figure 1: QKD system with integrated transmitter chip. (a) Schematic setup of the QKD system and the transmitter chip architecture. The chip consists of three high-speed Mach-Zehnder modulators (MZI-1 to MZI-3) and one thermal Mach-Zehnder modulator(VOA). MZI-1 is employed for decoy state preparation; MZI-2 and MZI-3 are for polarization encoding. VOA is used as an attenuator. The superscript $\pm$ inside the phase modulator indicates the upper and lower paths of the Mach-Zehnder modulator, respectively. (b) The opto-electronic packaging of the integrated photonic chip with compact printed circuit board, and (c) the microscope image of the chip corresponding to the red-boxed area.
• Figure 2: Laser source, SNSPD and transmitter chip. (a) The temporal profiles of the gain-switched laser with 100 measurements. (b) The scanning electron microscope (SEM) image of the twin-layer SNSPD chip. (c) Detection efficiency (blue, left axis) and time jitters (red, right axis) of the SNSPD with different input photon intensities. (d) Voltage-dependent transmission of the MZI and (e) loss of the carrier-depletion phase modulator. (f) The ERs of the MZI to the 5-GHz pulses by suppressing the even (blue)/odd (red) pulses.
• Figure 3: Concept diagram for perfect polarization state preparation. (a) The iterative trajectory curves of imperfect $|V'\rangle$ to the ideal $|V\rangle$ on the Poincaré sphere. (b) The error rate $R_{err}$ with respect to $\Delta\varphi_2$ and $\Delta\varphi_3$. A three-step iterative process is enough to approach a negligible error. (c)-(e) The iterative trajectory curves of $R_{err}$ from the original point A to the nearly perfect point D.
• Figure 4: SKR and stability of the QKD system. (a) SKR over distance. (b) Real-time SKR and QBERs for 6 hours. (c)-(e) Frequency statistics on SKR, QBERs of Z and X bases, respectively.