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IEEE Journal of Quantum Electronics

Generation of stable and high extinction ratio light pulses for continuous variable quantum key distribution

Xuyang Wang, Jianqiang Liu, Xuefeng Li, Yongmin Li

Published Jan 1, 2015• 25 citations
Publisher

Abstract

We propose and demonstrate an approach to generate stable and high extinction ratio light pulses with an extinction ratio 80 dB. To this end, a high stable bias locking technique is proposed, and a small size, easily integrated, cost-effective pulse generator based on delay line chips is designed and constructed. The pulse generator can generate voltage >12.5 V at a load impedance of 50 Ω, which is enough to drive the lithium niobate-based Mach-Zehnder (LNMZ) intensity modulator with a half wave voltage ~6-8 V. The pulse width and delay time can be programmed directly via digital I/O ports. The bias locking technique utilizes a successive scanning and fitting method which overcomes the sensitivity limit of the photodetector and can achieve the bias voltage accurately and quickly. By this method, the bias points of two or more cascaded LNMZ intensity modulators can be locked stably. The presented system can be easily integrated into the continuous variable quantum key distribution system.

DOI
10.1109/jqe.2015.2427031

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Abstract Quantum key distribution (QKD) can help distant agents to share unconditional secret keys, andthe achievable secret key rate can be enhanced with the help of decoy-state protocol. To implementQKD experimentally, the agents are supposed to accurately transmit a number of different intensitypulses with the LiNbO3 based Mach-Zehnder (LNMZ) intensity modulator. However, the bias driftof LNMZ intensity modulator may affect the performance of a QKD system. In this letter, we reveala simple RC circuit model to demonstrate the bias drift in the LNMZ intensity modulator. Andbased on the model, we propose a multi-step bias stable scheme to control the bias working point.Experimental result shows that our scheme can eliminate the bias drift of at arbitrary working pointwithin a long time range. Besides, there is no need of any feedback mechanisms in the scheme. Thismeans our scheme will not lead to any increasement in system complexity, making it more suitablefor a QKD system.

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We propose and demonstrate an approach to generate stable and high extinction ratio light pulses with an extinction ratio 80 dB. To this end, a high stable bias locking technique is proposed, and a small size, easily integrated, cost-effective pulse generator based on delay line chips is designed and constructed. The pulse generator can generate voltage >12.5 V at a load impedance of 50 Ω, which is enough to drive the lithium niobate-based Mach-Zehnder (LNMZ) intensity modulator with a half wave voltage ~6-8 V. The pulse width and delay time can be programmed directly via digital I/O ports. The bias locking technique utilizes a successive scanning and fitting method which overcomes the sensitivity limit of the photodetector and can achieve the bias voltage accurately and quickly. By this method, the bias points of two or more cascaded LNMZ intensity modulators can be locked stably. The presented system can be easily integrated into the continuous variable quantum key distribution system.
A Mach–Zehnder modulator (MZM) is necessary for implementing a decoy-state protocol in a practical quantum key distribution (QKD) system. However, an MZM bias control method optimized for QKD systems has been missing to date. In this study, we propose an MZM bias control method using N (≥2) diagnostic pulses. The proposed method can be efficiently applied to a QKD system without any additional hardware such as light sources or detectors. Furthermore, it does not reduce the key rate significantly because it uses time slots allocated to existing decoy pulses. We conducted an experimental demonstration of the proposed method in a field-deployed 1 × 3 QKD network and a laboratory test. It is shown that our method can maintain the MZM extinction ratio stably over 20 dB (bit error rate ≤1%), even in an actual network environment for a significant period. Consequently, we achieved successful QKD performances.
Mach-Zehnder modulators (MZMs) are the most commonly used external modulators for optical communication links. The bias point of MZMs may drift due to environmental changes in practical applications, leading to system performance degradation. The latest decoy-state twin-field quantum key distribution (TF-QKD) system requires multiple modulators to generate multi-level light intensities and maintain an ultra-high extinction ratio between the signal-state light and vacuum-state light. This is essential to ensure the QKD security and extend the transmission distance. To meet this tight requirement, this paper presents an efficient bias control technique of multistage MZMs based on the dither signal method. Considering the complex signal processing algorithms involved in the dither signal method, this work designs a high-throughput hardware architecture based on field programmable gate array (FPGA) platform to realize real-time feedback control. It is comprised of three modules: the dither generation module, the fast Fourier transform (FFT) module, and the bias control module. Both static and dynamic extinction ratio tests were conducted to verify the performance of the system. Especially, this paper proposes an innovative and dynamic measurement method based on the highly sensitive single-photon detector to support the direct extinction ratio test of high-speed signals. The experimental results show that the system can stably maintain an ultra-high extinction ratio of 66.92 dB, demonstrating itself suitable for the majority of optical communication systems with very tight extinction ratio requirements. The feedback control time is about only 0.5 seconds per stage, which is fast enough to compensate for the usual environmental drift. In the temperature stability experiments, when the temperature varies between 26-64°C, the proposed system achieves a 74% reduction of the extinction ratio fluctuation compared to the result without using the feedback system.
A quantum key distribution transmitter chip based on hybrid-integration of silica planar light-wave circuit (PLC) and lithium niobates (LN) modulator PLC is presented. The silica part consists of a tunable directional coupler and 400-ps delay line, and the LN part is made up of a Y-branch, with electro-optic modulators on both arms. The two parts are facet-coupled to form an asymmetric Mach–Zehnder interferometer. We successfully encode and decode four BB84 states at 156.25-MHz repetition rate. Fast phase-encoding of 0 or π is achieved, with interference fringe visibilities 78.53% and 82.68% for states |+〉 and |–〉, respectively. With the aid of an extra off-chip LN intensity modulator, two time-bin states are prepared and the extinction ratios are 18.65 dB and 15.46 dB for states |0〉 and |1〉, respectively.
Quantum key distribution allows secret key generation with information theoretical security. It can be realized with photonic integrated circuits to benefit the tiny footprints and the large-scale manufacturing capacity. Continuous-variable quantum key distribution is suitable for chip-based integration due to its compatibility with mature optical communication devices. However, the quantum signal power control compatible with the mature photonic integration process faces difficulties on stability, which limits the system performance and causes the overestimation of a secret key rate that opens practical security loopholes. Here, a highly stable chip-based quantum signal power control scheme based on a biased Mach-Zehnder interferometer structure is proposed, theoretically analyzed, and experimentally implemented with standard silicon photonic techniques. Simulations and experimental results show that the proposed scheme significantly improves the system stability, where the standard deviation of the secret key rate is suppressed by an order of magnitude compared with the system using traditional designs, showing a promising and practicable way to realize a highly stable continuous-variable quantum key distribution system on chip.
Conventional continuous-variable quantum key distribution systems typically rely on discrete optical components, which have limited integration, are bulky and expensive. To overcome these drawbacks, photonic integrated circuit technology is a viable alternative that promises to increase the system integration. However, variable optical attenuators compatible with established photonic integration processes have stability difficulties that limit the performance of the system. This work provides a chip-based variable optical attenuator based on Mach-Zehnder interferometer structure. By using an unbalanced interferometer arm, the sensitivity of the optical attenuation coefficient to environmental fluctuations can be reduced, ensuring the high-precision control required for continuous-variable quantum key distribution systems. Our scheme will facilitate the implementation of a reliable and highly stable chip-based continuous-variable quantum key distribution system.
Abstract Quantum key distribution (QKD) can help distant agents to share unconditional secret keys, andthe achievable secret key rate can be enhanced with the help of decoy-state protocol. To implementQKD experimentally, the agents are supposed to accurately transmit a number of different intensitypulses with the LiNbO3 based Mach-Zehnder (LNMZ) intensity modulator. However, the bias driftof LNMZ intensity modulator may affect the performance of a QKD system. In this letter, we reveala simple RC circuit model to demonstrate the bias drift in the LNMZ intensity modulator. Andbased on the model, we propose a multi-step bias stable scheme to control the bias working point.Experimental result shows that our scheme can eliminate the bias drift of at arbitrary working pointwithin a long time range. Besides, there is no need of any feedback mechanisms in the scheme. Thismeans our scheme will not lead to any increasement in system complexity, making it more suitablefor a QKD system.