Integrated Photonic Technologies for Quantum Communications

Photonics has been fundamental in the development of quantum information and communication technologies, as well as providing insight into the fundamental mechanisms of nature. Quantum photonic experiments have had a central role in the advancement of quantum technologies, with single and multi-photon experiments being performed in bulk optical, fibre optic, and more recently integrated photonic platforms. Integrated photonics has provided the miniaturisation, manufacturability and reconfigurability required for demanding applications within classical telecommunication and photonic technologies. and has recently been adopted in many quantum technologies, including quantum computing and metrology. The inherent stability, miniaturisation and reconfigurability has led to many experiments, otherwise impractical, in a manufacturable and scalable manner. Recent demonstrations include; quantum sensing with integrated photonics [1], on chip generation and manipulation of photons for quantum information processing in silicon [2], integrated single photon detectors [3], quantum simulation in silica circuits [4], and numerous quantum computation tasks on a single reconfigurable photonic processor [5]. Quantum Key Distribution (QKD) has developed over the last few decades from proof-of-principle experiments to robust long range demonstrations in free-space and fibre optics, and even work towards satellite communications. Previously many demonstrations of quantum secured communication have utilised integrated components or processes for the benefits of miniaturisation, manufacturability, or complexity. Electro-optic modulators have provided multiprotocol operation [6], planar lightwave circuitry has been used for time-bin decoding asymmetric interferometers [7], and lithium niobate polarisation modulators have been used for miniaturised chip “clients” [8]. Recent work in Bristol has led to the development of integrated photonic transmitters and receivers for high speed, multi-protocol QKD, utilising commercial fabrication facilities (Figure 1). This has exploited indium phosphide for the monolithic integration of laser diodes, high-speed electro-optic phaser modulation, and monitoring photodiodes, to prepare multi-protocol weak coherent time-bin encoded states, for communication over optical fibre. The receiver circuitry is fabricated from silicon oxynitride with thermo-optical reconfigurability, and off-chip superconducting single photon detectors, providing high efficiency, low noise, and low jitter measurement. The results are comparable to state-of-the-art performance, with high clock rates of 1.7 GHz, low quantum bit error rates of 0.88%, and estimated secret key rates up to 568 kbs−1 for an emulated 20 km fibre link, demonstrating the feasibility of using integrated photonic circuitry for quantum communication applications, with a low cost, small footprint, and flexible circuitry [9]. The coherent distribution and manipulation of quantum information between quantum circuits would further facili-