A hybrid integrated quantum key distribution transceiver chip

www.nature.com/npjqi ARTICLE OPEN A hybrid integrated quantum key distribution transceiver chip Joseph A. Dolphin 1,2 ✉ , Taofiq K. Paraïso 1 , Han Du 1 , Robert I. Woodward 1 , Davide G. Marangon1 and Andrew J. Shields1 Quantum photonic technologies, such as quantum key distribution, are already benefiting greatly from the rise of integrated photonics. However, the flexibility in design of these systems is often restricted by the properties of the integration material platforms. Here, we overcome this choice by using hybrid integration of ultra-low-loss silicon nitride waveguides with indium phosphide electro-optic modulators to produce high-performance quantum key distribution transceiver chips. Access to the best properties of both materials allows us to achieve active encoding and decoding of photonic qubits on-chip at GHz speeds and with sub-1% quantum bit error rates over long fibre distances. We demonstrate bidirectional secure bit rates of 1.82 Mbps over 10 dB channel attenuation and positive secure key rates out to 250 km of fibre. The results support the imminent utility of hybrid integration for quantum photonic circuits and the wider field of photonics. 1234567890():,; npj Quantum Information (2023)9:84 ; https://doi.org/10.1038/s41534-023-00751-3 INTRODUCTION Methods of data encryption are in constant competition with the abilities of attackers hoping to decrypt them. In an age of everincreasing data exchange and the rising threat from quantum computation, quantum key distribution (QKD) provides a method of symmetric key exchange based on the fundamental principles of quantum mechanics1,2. By encoding quantum information into the states of single photons, a secure key can be exchanged between two parties with information-theoretic security. Over the last 40 years this technology has matured from a theoretical concept to field trials and commercial products3–6. However, the hardware required for QKD remains bulky and expensive7, which threatens to restrict QKD to only the most cost-insensitive of applications. It has thus been recognised that a move towards integrated photonics is an appealing option for QKD. Miniaturisation of the optical components into photonic integrated circuits (PICs) promises to increase the manufacturability of QKD systems to a point where widespread adoption becomes more feasible. In recent years, these ‘on-chip’ QKD systems have been demonstrated in a wide variety of formats, such as time-bin encoding8–16, polarisation encoding17–23, free-space channel22, measurementdevice independent QKD13,19,20 and continuous-variable QKD24. Recent efforts have achieved milestones such as integrated detectors15, a complete real-time chip-based QKD system based on pluggable modules14, combined electronic and photonic integration16,21, and an integrated transmitter used in the demonstration of the highest QKD secure key rate to date25. However, the performance of QKD-PICs is constrained by the properties of the material platforms currently available, particularly in the case of the quantum receiver circuits. Here, any optical losses directly subtract from final secure key rates, making receiver loss a key performance indicator for the overall system. Low-loss receiver circuits can be produced from materials such as silica (SiO2), silicon nitride (SiN) or silicon oxynitride (SiOxNy), but these platforms lack high-speed modulators. Receiver circuits without high-speed modulators must rely on passive structures to measure orthogonal measurement bases. This typically comes with downsides, such as the need for increased detector numbers or the need to use higher degrees of freedom, such as time-of-arrival. Indium phosphide (InP), a commonly used PIC platform with effective high-speed electro-optic phase modulators (EOPMs), suffers from relatively high waveguide propagation losses (1–4 dBcm−1)26 which discourages its use in receivers. The only platform combining low propagation losses with high-speed modulation is silicon photonics. However, a weak electro-optic effect has meant that for high-speed modulation silicon photonics usually relies on plasma dispersion effects, such as carrier depletion modulators. In recent years work on carrier depletion modulators has increased the achievable bandwidths above 50 GHz27, but they still exhibit comparatively poor performance with regards to baseline loss, phase-dependent loss10 and phasemodulation efficiency when compared to EOPMs28,29. To date, the majority of on-chip QKD demonstrations have opted for passive receivers, with no examples of active on-chip receivers operating above 10 MHz. As a solution to the limitations of the available PIC platforms we identify hybrid integration, where two different materials are combined in one photonic circuit. Others have already identified hybrid integration as an important next step for quantum integrated photonics30,31. Indeed, for classical integrated photonics, hybrid integration has already emerged as an important capability. This has usually come in the context of integrating III/V lasers onto silicon photonics, which does not possess a native lasing ability. A number of different hybrid integration approaches have been developed, including flip-chip32, edge-coupling (also called butt-coupling)33, and photonic wire bonding34, as well as heterogeneous methods35,36. However, to date the application of hybrid integration to quantum integrated photonics has been notably limited37,38, with no full demonstrations of QKD. In this work, we present the results from a prototype edgecoupled hybrid QKD transceiver. By combining an ultra-low-loss SiN interferometer with the superior modulation characteristics of an InP EOPM, we demonstrate actively modulated transmitter and receiver circuits with low insertion loss (<8 dB), low operating voltage (4 V) and high clock rate (1 GHz). We measure reliably < 1 dB interface loss between the SiN and InP PICs, a value that compares well to other hybrid integration approaches and supports the use of edge-coupling when low insertion loss is 1 Toshiba Europe Ltd, 208 Cambridge Science Park Milton Rd, Cambridge CB4 0GZ, UK. 2Department of Engineering, University of Cambridge, Trumpington St, Cambridge CB2 1PZ, UK. ✉email: Published in partnership with The University of New South Wales J.A. Dolphin et al. 1234567890():,; 2 Fig. 1 An overview of the hybrid transceiver experiment. a A schematic diagram of the experimental setup including hybrid chip circuits. Two hybrid PICs are shown connected via a duplex fibre channel of either emulated loss or real fibre spools. The visible on-chip components include EOPMs, mach-Zehnder interferometers (MZIs), thermo-optic phase modulators (TOPMs) and delay lines (DLs). Variable optical attenuators (VOAs) are used to bring the photon flux down to single photon levels. Polarisation controllers (PC) are used to align the axis of polarisation between PICs. Each hybrid PIC is connected to an externa (...truncated)