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Quantum 2.0 Conference and Exhibition

Advanced Laser Modulation and Chip-Based Quantum Communications

Taofiq K. Paraïso, Thomas Roger, Davide G. Marangon, Innocenzo De Marco, Mirko Sanzaro, Robert I. Woodward, J. F. Dynes, V. Lovic, Zhiliang L. Yuan, Andrew J. Shields

Published Jan 1, 2022• 0 citations
Publisher

Abstract

We review recent implementations of quantum key distribution transmitters based on optical injection locking and direct modulation. By appropriately harnessing laser properties we show how these developments led to the realization of practical chip-based quantum key distribution systems.
DOI
10.1364/quantum.2022.qtu3b.3

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Abstract Quantum communications aim to share encryption keys between the transmitters and receivers governed by the laws of quantum mechanics. Integrated quantum photonics offers significant advantages of dense integration, high stability and scalability, which enables a vital platform for the implementation of quantum information processing and quantum communications. This article reviews recent experimental progress and advances in the development of integrated quantum photonic devices and systems for quantum communications and quantum networks.
Quantum key distribution (QKD) is a technology that allows two users to communicate with theoretically perfect security using standard optical fibres. This is possible by transmitting the key on single photons, meaning a measurement by an eavesdropper disturbs the system in a way observable to the legitimate parties. The technology has advanced since the first protocol proposed in 1984, to the stage where there are now many protocols that can be experimentally implemented. These protocols have allowed secure keys to be generated over distances greater than 400 km and with secure key rates over 10 Mbit/s. In a metropolitan QKD network, it would be desirable for as many users to be connected as possible. Unfortunately, each protocol comes with different requirements on the transmitter and receiver. Even within a single protocol, different clock rates can require individualised transmitter and receiver hardware. This prohibits users from communicating with all receivers, unless they have complex transmitters with hardware for many protocols. This thesis develops a transmitter for practical QKD that is able to adapt to a number of different protocols with no changes to the hardware. The transmitter works using optical injection locking, where a pulse preparation laser adopts the phase of a phase preparation laser. Controlling the phase and intensity of the pulses in this way removes the side channel ordinarily present with direct modulation, in that the phase, intensity and frequency simultaneously change in response to an applied current. The cavity-enhanced electro-optic effect allows for the first demonstration of sub-volt half-wave phase modulation at high clock rates. The transmitter successfully demonstrates phase encoding, intensity encoding and on-demand phase randomisation. This allows for the experimental realisation and direct comparison of different QKD protocols, including one that has never before been implemented due to experimental complexity. A stable intensity modulator is also developed, based on a Sagnac interferometer. This removes a side channel in QKD systems and integrates well with the directly-modulated quantum transmitter. This development also means that the transmitter can implement all current two-party QKD protocols based on weak coherent pulses. The transmitter has the potential to become the standard transmitter for future quantum communication networks due to its stability, versatility and power efficiency. The design could also be demonstrated on a photonic chip, making it compact enough to fit in small transmitter units.
For the adoption of QKD to grow, much effort has been devoted to making QKD systems more robust and efficient. Much of the complexity of a QKD system stems from its transmitter where quantum states encoded with bit values are prepared. Recently, optical injection locking (OIL) has emerged as a promising method to realize high-speed QKD transmitters with a compact design. This approach enables direct phase encoding without the need for external modulators, while simultaneously improving the laser characteristics. Due to these remarkable advantages, OIL has been widely applied to many QKD protocols, including BB84, MDI-QKD and TF-QKD. However, in practice, tuning the laser system to find optimal operating parameters is a very challenging task. This is because the underlying laser dynamics are rich and involve a complex interplay between multiple control parameters. It is therefore highly desirable to develop an efficient method to optimize the systems. Here, for the first time, we address this issue by demonstrating a self-tuning QKD transmitter by implementing a genetic algorithm to autonomously locate the optimum system parameters. Without any user intervention, our approach manages to optimize the quantum bit error rate down to ~2.5%, matching the state-of-the-art performance.