Optical fuse based on the photorefractive effect for defending the light-injection attacks of quantum key distribution

TL;DR

The paper addresses the vulnerability of QKD systems to light-injection attacks and the limitations of conventional defenses. It proposes an integrated optical fuse based on the photorefractive effect in a thin-film lithium niobate microring resonator to sense attacks and automatically respond on-chip. The authors show two defense modes: resonant attack light blue-shifts the MRR to suppress transmission, and non-resonant light is rejected with a high isolation, including a broadband response tested in a commercial BB84 system. The results demonstrate microwatt-level attack sensitivity with substantial key-rate suppression under attack but maintained defensive capability across a wide spectral range, highlighting the practical potential for on-chip QKD security improvements and applicability to MDI-QKD and CV-QKD.

Abstract

Light-injection attacks pose critical security threats to quantum key distribution (QKD) systems. Conventional defense methods, such as isolators, filters, and optical power monitoring, are confronted with the threats of specific attacks and the limitations in integration. To address this, we propose and experimentally demonstrate an integrated attack sensing and automatic response unit utilizing the photorefractive effect in a thin-film lithium niobate microring resonator. Our unit provides a high rejection ratio against non-resonant injected light. For resonant attacks exceeding tens of microwatts, the unit can autonomously attenuate the transmission of the quantum signal light, leading to a significant suppression of the secret key rate. This work enhances the security of QKD systems against light-injection attacks by providing a highly sensitive, broadband, and on-chip defense mechanism.

Figures (5)

• Figure 1: (a) Operational principle of the optical fuse in a QKD system. The optical fuse is placed at the transmitter output to protect against light-injection attacks. (i) Resonant attack mode: attack light at the MRR resonance wavelength induces a blue shift in the MRR spectrum via the PR effect, thereby attenuating signal transmission. (ii) Non-resonant attack mode: the unit functions as a filter that blocks the attack light while maintaining normal signal light propagation. (b) Illustration of the PR effect in a TFLN waveguide, where $E_{\text{SC}}$ denotes the space-charge field.
• Figure 2: (a) The microscope photo of the MRR and the cross-sectional schematic of the waveguide. (b) Transmission spectrum of the MRR with a zoomed-in view, showing a load quality factor $Q_{\text{load}} = \qty{6.6e4}{}$, and an FSR of 50GHz for unit. (c) Schematic of the experimental setup. Laser 1, a $\qty{1550}{nm}$ signal laser. PC, polarization controller. VOA, variable optical attenuator. BS, beam splitter, a $1:99$ beam splitter. PM, power meter A. Cir, a fiber circulator. Optical Fuse, the unit under test. WDM, wavelength division multiplexer. EDFA, erbium-doped fiber amplifier. Laser 2, a tunable $\qty{1550}{nm}$ attack laser.
• Figure 3: (a) Transmission spectrum shifts under different resonant attack powers. (b) Signal transmission attenuation of 1550.68nm CW light under different resonant on-chip attack powers. (c) Temporal evolution of signal transmission under different attack powers, with the attack initiated at $t = \qty{5}{s}$ and terminated at $t = \qty{65}{s}$. (d) Signal transmission (red curve) and corresponding attack power reaching Alice (blue curve) under different non-resonant attack powers.
• Figure 4: (a) Experimental schematic for testing in a commercial QKD system. The attack module (identical to Fig. \ref{['Fig.2']}(c)) intercepts the quantum channel, and the optical fuse is deployed at the transmitter output. (b) Transmission attenuation of the signal pulse under resonant attack. The width of the signal pulse is 100ps. (c) Signal pulse transmission attenuation (blue curve) and the on-chip attack light power (red curve) under various attack light wavelengths. The wavelength is tuned over one FSR of the unit in 20pm steps, centered at the resonant wavelength $\qty{1548.292}{nm}$. The incident attack power reached transmitter was fixed at -20dBm.
• Figure 5: Simulation and experimental SKR under resonant attack at different attack powers. (a) SKR versus resonant attack power at a fixed distance of 30km. (b) SKR versus transmission distance under different attack powers.