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Optics Express

Silicon photonic integrated QKD transceiver utilizing an external cavity laser as the light source

Yu Zhou, Yuyao Guo, Yan-yang Zhou, Y. P. Lu, Xiao-Lei Jiang, Liangjun Lu, Linjie Zhou, Wan-Su Bao

Published Feb 5, 2026• 0 citations
Publisher

Abstract

Integrated silicon photonics offers a promising route toward compact and CMOS-compatible quantum key distribution (QKD) systems. However, on-chip laser integration remains a key barrier to full system miniaturization. Here, we demonstrate a fully integrated photonic device that combines an external-cavity laser (ECL) with a QKD transceiver, enabled by hybrid III-V/silicon photonic integration. The integrated transmitter and the phase-decoding receiver are combined to form a phase-encoded QKD system that demonstrates quantum bit error rates (QBER) of 1.4% (Z-basis) and 6.4% (X-basis), and a secure key rate of 28.16 kbps over a fiber channel with 8.3 dB loss at 625 MHz. The integrated system achieves a wavelength tunability of 87.61 nm, addressing the challenge of light source integration in silicon-based QKD networks. The demonstrated integration platform represents a crucial step toward the practical integration of QKD into existing optical transport networks (OTNs) and future quantum communication networks.

DOI
10.1364/oe.582305

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Integrated silicon photonics offers a promising route toward compact and CMOS-compatible quantum key distribution (QKD) systems. However, on-chip laser integration remains a key barrier to full system miniaturization. Here, we demonstrate a fully integrated photonic device that combines an external-cavity laser (ECL) with a QKD transceiver, enabled by hybrid III-V/silicon photonic integration. The integrated transmitter and the phase-decoding receiver are combined to form a phase-encoded QKD system that demonstrates quantum bit error rates (QBER) of 1.4% (Z-basis) and 6.4% (X-basis), and a secure key rate of 28.16 kbps over a fiber channel with 8.3 dB loss at 625 MHz. The integrated system achieves a wavelength tunability of 87.61 nm, addressing the challenge of light source integration in silicon-based QKD networks. The demonstrated integration platform represents a crucial step toward the practical integration of QKD into existing optical transport networks (OTNs) and future quantum communication networks.
Abstract 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.
Quantum key distribution (QKD) provides information-theoretic security independent of computational assumptions, yet the bulk and cost of current systems hinder large-scale deployment. Although integrated photonic technologies have enabled highly integrated QKD chips, practical QKD transmitters are still predominantly implemented as rack-mounted systems. Here, we demonstrate a highly integrated standalone QKD transmitter that integrates all essential functionalities required for practical QKD operation within a compact platform built around a silicon photonic encoding chip. The transmitter occupies only 167×56×21 mm3167 \times 56 \times 21~\mathrm{mm}^3167×56×21 mm3 (∼0.2 dm3\sim 0.2~\mathrm{dm}^3∼0.2 dm3), comparable in size to a half-height, half-width PCIe card and more than 30 times smaller in volume than a conventional 1U rack-mounted system. Paired with a conventional discrete-component receiver, it achieves a secure key rate of 219.1 kbps over 51.3 km of standard single-mode fiber, with performance comparable to that of conventional discrete-component implementations. This work bridges the gap between photonic chip integration and deployable QKD hardware, marking an important step toward transitioning QKD transmitters from conventional rack-mounted equipment to compact board-level platforms.
Integrated photonics provides a route to both miniaturization of quantum key distribution (QKD) devices and enhancing their performance. A key element for achieving discrete-variable QKD is a single-photon detector. It is highly desirable to integrate detectors onto a photonic chip to enable the realization of practical and scalable quantum networks. We realize a heterogeneously integrated, superconducting silicon-photonic chip. Harnessing the unique high-speed feature of our optical waveguide-integrated superconducting detector, we perform the first optimal Bell-state measurement (BSM) of time-bin encoded qubits generated from two independent lasers. The optimal BSM enables an increased key rate of measurement-device-independent QKD (MDI-QKD), which is immune to all attacks against the detection system and hence provides the basis for a QKD network with untrusted relays. Together with the time-multiplexed technique, we have enhanced the sifted key rate by almost one order of magnitude. With a 125-MHz clock rate, we obtain a secure key rate of 6.166 kbps over 24.0 dB loss, which is comparable to the state-of-the-art MDI-QKD experimental results with a GHz clock rate. Combined with integrated QKD transmitters, a scalable, chip-based, and cost-effective QKD network should become realizable in the near future.
Silicon (Si) photonics is reaching maturity and has the potential of offering low-cost, reliable, chip-scale systems for quantum key distribution (QKD). On the other hand, QKD is also maturing with the bridging of theory and practice. This work presents a Si integrated photonic transmitter for polarization-encoded QKD. The chip was fabricated in a standard Si photonic foundry process and integrates a pulse generator, intensity modulator, variable optical attenuator, and polarization modulator in a 1.3 mm x 3 mm die area. The devices in the photonic circuit meet the requirements for QKD. The transmitter was used in a proof-of-concept demonstration of the BB84 QKD protocol over a 5 km long fiber link. A quantum bit error rate of 5.4% and an asymptotic secure key rate of 0.95 kbps is obtained. This work shows the feasibility of foundry based Si photonics for quantum information applications.
Continuous-variable quantum key distribution (CVQKD) is a promising technology for enabling quantum-secure networks, valued for its compatibility with conventional telecommunication hardware and maturing security proofs. Although the need for quantum-safe infrastructure is driven by the threat of advancing quantum computing, the widespread deployment of QKD remains elusive because current discrete-component solutions suffer from high cost, high energy consumption, and lack of conformity with the existing telecom network infrastructure. In this work, we demonstrate a compact CV-QKD system based on a custom-designed, monolithically integrated silicon photonic dualpolarization transceiver, operated with an off-chip seed laser. Taking into account the block length of 2 × 107 and employing 64-quadrature amplitude modulation (QAM) shaped probabilistically, our system achieves a secret key rate of 1.9 Mbit/s in 25km of standard single-mode fiber. This work highlights the significant potential of electronic-photonic integration to advance the practical deployment of QKD networks.
Quantum key distribution (QKD) allows the secure transmission of encryption keys in the dawning age of quantum computers. However, despite photonic integration efforts, the size and complexity of current QKD systems remains a roadblock for practical deployment in cost-sensitive applications. Here we show a monolithic integrated QKD transmitter that exclusively builds on silicon photonics. We introduce a germanium-on-silicon light emitter acting as source for a co-integrated quantum state encoder, to generate a secure key over a 45.9 km field-installed fiber link. On top of this, we demonstrate the broadband operation of our QKD transmitter by generating keys over 32 wavelength channels, making it an integrated colorless QKD transmitter with full C-band coverage. Employing monolithic silicon technology paves the way towards fully integrated optoelectronic QKD transmitters, enabling a drastic reduction of size and cost while benefiting from the maturity and mass production capabilities of complementary metal–oxide semiconductor (CMOS) technology. Moreover, omitting the need for rare and costly III-V semiconductor materials to generate light can drastically reduce the overburdening assembly and packaging requirements inherent to optoelectronics, potentially enabling QKD to finally enter new application domains closer to the consumer market.