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Optical Fiber Communication Conference

Photonic Integrated Chip-based Reference Frame Independent Quantum Key Distribution Transmitter

Kyongchun Lim, Byung‐Seok Choi, Ju Hee Baek, Minchul Kim, Joong‐Seon Choe, Kap‐Joong Kim, Dong Churl Kim, Junsang Oh, Chun Ju Youn

Published Jan 1, 2025• 0 citations
Publisher

Abstract

For the first time, we present a photonic integrated chip-based RFI QKD transmitter, modularized in CFP2 form factor. The transmitter’s capability is demonstrated through free-space QKD experiments, highlighting its potential for secure quantum communication.
DOI
10.1364/ofc.2025.th3i.4

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Deployable quantum key distribution (QKD) for free-space link must be both reference frame tolerant and hardware compact, yet existing free-space implementations still rely on bulk optics and active alignment. To address this gap, we develop the first reference frame independent (RFI) QKD transmitter fully integrated on a photonic-integrated circuit (PIC) and packaged in a CFP2 optical-transceiver form factor. The PIC incorporates a six-channel laser diode array, a six-channel variable optical attenuator array, and a polarization beam combiner, providing wavelength indistinguishability and high polarization fidelity. Combined with a micro-optics based polarization decoder and a bidirectional two-stage pointing, acquisition, and tracking (PAT) system, the PIC transmitter forms a complete free-space RFI QKD system. Based on the system, RFI QKD is performed with 10 m free-space channel, providing that QBERs measured in the same basis are below 3.81 % and the security parameter is1.77±0.021.77\pm 0.021.77±0.02. This leads a stable secret key rate of77.33±1.65kbps77.33\pm 1.65 \text{kbps}77.33±1.65kbpsfor 100 s. These results verify the feasibility of PIC-based RFI QKD transmitter and establish a clear pathway toward fully deployable PIC-based QKD systems.
Quantum key distribution (QKD) is moving from research laboratories towards applications. As computing becomes more mobile, cashless as well as cardless payment solutions are introduced. A possible route to increase the security of wireless communications is to incorporate QKD in a mobile device. Handheld devices present a particular challenge as the orientation and the phase of a qubit will depend on device motion. This problem is addressed by the reference frame independent (RFI) QKD scheme. The scheme tolerates an unknown phase between logical states that vary slowly compared to the rate of particle repetition. Here we experimentally demonstrate the feasibility of RFI QKD over a free-space link in a prepare and measure scheme using polarization encoding. We extend the security analysis of the RFI QKD scheme to be able to deal with uncalibrated devices and a finite number of measurements. Together these advances are an important step towards mass production of handheld QKD devices.
Reference-frame-independent quantum key distribution (RFI-QKD) provides a practical way to generate secret keys between two remote parties without sharing common reference frames. On the other hand, measurement-device-independent QKD (MDI-QKD) offers a high level of security, as it is immune to all quantum hacking attempts to measurement devices. The combination of these two QKD protocols, i.e., RFI-MDI-QKD, is one of the most fascinating QKD protocols, since it holds advantages of both practicality and security. For further practicality of RFI-MDI-QKD, it is beneficial to reduce the implementation complexity. Here, we show that RFI-MDI-QKD can be implemented using fewer quantum states than those of its original proposal. We find that, in principle, the number of quantum states for one of the parties can be reduced from six to three without compromising security. Compared to conventional RFI-MDI-QKD where both parties transmit six quantum states, it significantly simplifies the implementation of the QKD protocol. We also verify the feasibility of the scheme with a proof-of-principle experiment.
A global quantum secure network (QSN) based on quantum key distribution (QKD) has emerged as a significant trend since it provides an unconditionally secure way to defeat the threat posed by quantum computers. The nodes, based on ground or mobile platforms, are connected with two different channels, free-space and fiber. In this paper, a qubit-based synchronization reference-frame-independent (RFI) QKD is demonstrated in the field with a free-space/fiber integrated channel, in which the bright classical reference-frame calibration and clock synchronization signals are removed, and the quantum state is directly converted between two different channels without extra coding/decoding operations. It not only reduces the complexity of QKD and QSN but also enhances the practical security (e.g., removing calibration attacks and trusted repeaters). A key rate of 799.2 bps is achieved under 7 km urban free-space and 75 km fiber link. And the long time tests of a continuous 12 h without human intervention also demonstrate the stability of our system under such an integrated channel. Thus, we believe this work opens a way to implement a simple, flexible, and low-cost QSN.
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.
Photonic chip integration would greatly benefit quantum key distribution (QKD) systems for secure communication, in terms of compactness, scalability, and well-established fabrication techniques. This study demonstrates proof of concept for an all-chip measurement-device-independent QKD system using integrated silicon photonic technology. The prototype generates a key rate per pulse of $2.923\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ over a distance corresponding to 50 km of standard fiber with 25% detection efficiency; the system is projected to be able to work at 120 km with 85% detection efficiency. This technology shows great promise for highly integrated quantum communication networks.