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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Photonic Integrated Circuit for Compact Quantum Key Distribution

Martin Achleitner, Aaron Stadler, Marie-Christine Roehsner, Mariana F. Ramos, Julian Konig, Ozan Çirkinoglu, Xaveer J. M. Leijtens, Hannes Hübel

Published Jun 23, 2025• 0 citations
Publisher

Abstract

For the establishment of secure communication as well as a future quantum internet, technologies like Quantum Key Distribution (QKD) must become smaller in size, cost-effective in production and more performant. Since attacks like store-now-decrypt-later are already a threat for sensitive data, research in this area must be pushed towards a higher practicality. In this work we show the feasibility of using a photonic integrated circuit (PIC) for application as a QKD transmitter. The PIC (provided by Eindhoven University of Technology) comprises a ring laser source, a Mach-Zehnder interferometer (MZI) with integrated RF phase modulators used for pulse carving and phase modulation as well as an attenuator section. This indium phosphide (InP) chip enables us to generate the necessary states for a 4-state time-bin-phase BB84 protocol [1] implementation. After characterization of all the photonic components and preliminary testing, we were able to generate a secret key rate of approximately 89kbps for 50MHz signal repetition rate with a QBER of 1.3% (in Z-Basis) in a back-to-back configuration.

DOI
10.1109/cleo/europe-eqec65582.2025.11111025

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For the establishment of secure communication as well as a future quantum internet, technologies like Quantum Key Distribution (QKD) must become smaller in size, cost-effective in production and more performant. Since attacks like store-now-decrypt-later are already a threat for sensitive data, research in this area must be pushed towards a higher practicality. In this work we show the feasibility of using a photonic integrated circuit (PIC) for application as a QKD transmitter. The PIC (provided by Eindhoven University of Technology) comprises a ring laser source, a Mach-Zehnder interferometer (MZI) with integrated RF phase modulators used for pulse carving and phase modulation as well as an attenuator section. This indium phosphide (InP) chip enables us to generate the necessary states for a 4-state time-bin-phase BB84 protocol [1] implementation. After characterization of all the photonic components and preliminary testing, we were able to generate a secret key rate of approximately 89kbps for 50MHz signal repetition rate with a QBER of 1.3% (in Z-Basis) in a back-to-back configuration.
In this work, an indium phosphide (InP) based quantum key distribution (QKD) transmitter on a photonic integrated circuit (PICs) is examined. The PIC, which was designed by the authors from Eindhoven University of Technology, and characterised in the QKD system by the authors from Austrian Institute of Technology, features a ring laser structure, a single Mach-Zehnder modulator (MZM) structure with integrated radio frequency (RF) phase modulators for pulse carving, intensity modulation and phase modulation and an absorber for attenuation. The individual components were characterised. Additionally, the PIC was tested in a 4-state time-bin BB84 protocol including a bulk receiver, key performance measures like quantum bit error rate (QBER) and secret key rate (SKR) were analysed. The high performance PIC is capable of generating a SKR in the kbits/s range exhibiting high stability over a measured time frame of around an hour.
Developing quantum key distribution (QKD) systems using monolithic photonic integrated circuits (PICs) can accelerate their adoption by a wide range of markets, thanks to the potential reduction in size, complexity of the overall system, power consumption, and production cost. In this work, we design, fabricate and characterize an InP-based PIC transmitter for continuous-variable (CV) QKD applications. In a proof-of-principle experiment implementing a pulsed Gaussian-modulated coherent state (GMCS) CV-QKD protocol over an optical fiber channel of 11 km, the system showed a performance compatible with a secret key rate of 78 kbps in the asymptotic regime. These results show the potential of InP technologies to integrate CV-QKD systems onto a monolithic platform.
Advances in quantum computing pose significant threats to the security of conventional encryption standards. Although quantum key distribution systems offer inherent security, they face challenges in achieving practical secret-key rates over long transmission distances. Photonic Integrated Circuit technologies provide scalability and system efficiency for high key rates using multiplexed time-bin protocols. We present the architecture of a 16-channel multiplexed QKD system at a GHz clock rate realized on integrated photonic chips, promising further scalability. The sender module revolves around an indium-phosphide chip featuring tunable lasers, modulators, and combiners. On the receiver side, low fiber-to-chip coupling loss and waveguide-integrated superconducting nanowire single-photon detectors on silicon nitride provide the necessary detection efficiency.
Quantum key distribution (QKD) is a quantum communication technology that offers absolute security. Among its various implementations, continuous-variable (CV) QKD has garnered significant attention due to its high secret key generation rates and compatibility with conventional optical telecommunication infrastructure, which operates under ambient conditions. However, chip-based integration is crucial for realizing practical and scalable CV-QKD networks, and photonic integrated circuit technology is a potential solution, leveraging the on-chip integration of essential optical components onto silicon photonic circuits. Therefore, this work presents a compact, stable, and cost-effective quantum photonic CV-QKD system on a chip. Our architecture combines time- and polarization-multiplexing with high-extinction-ratio pulse generation to mitigate excess noise. At the same time, a real-time feedback control mechanism compensates for polarization drift, ensuring robust system performance. Fiber-based evaluations reveal secret key generation rates of 2.55 Mbps over 25 km and 0.737 Mbps over 50 km with 12 h stable operation, establishing, to our knowledge, a new benchmark for integrated photonic CV-QKD systems with both the transmitter and receiver on-chip. These results mark a significant milestone in on-chip quantum communication, paving the way for widespread adoption of photonic integrated circuits in future metropolitan quantum-secure networks.
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.
Photonic integrated circuits (PICs) are key in advancing quantum technologies for secure communications. They offer inherent stability, low losses and compactness compared to standard fiber-based and free-space systems. Our reasearch demonstrates PIC's effectivness in enhancing quantum communications, implementing a three-state BB84 protocol with decoy-state method. We employ an integrated receiver and superconducting nanowire single photon detectors (SNSPDs) to achieve technological advancements. One of the most notable results is the extraction of a secret key over a record-breaking 45 dB channel attenuation. Our results demonstrate a remarkable 220% boost in key rate compared to our prototype fiber-based receiver over a 10 dB channel attenuation. This improvement in the secret key rate (SKR) signifies the potential of integrated photonics to advance the field of quantum communication.