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EPJ Quantum Technology

Optimizing path-polarization interconversion on a photonic circuit for secure quantum communication

Kap‐Joong Kim, Kyongchun Lim, Byung‐Seok Choi, Wook‐Jae Lee, Jong-Bum You, Young‐Ho Ko, Joong‐Seon Choe, Minchul Kim, Jeongho Bang, Joonwoo Bae, Chun Ju Youn

Published Jul 28, 2026• 0 citations
Publisher

Abstract

In this work, we consider an on-chip silicon device that prepares photonic qubits in mutually unbiased bases (MUBs), which can be used to realize quantum key distribution protocols. We present a realistic approach to dealing with generic imperfections in on-chip devices for generating MUBs, in particular, imperfections originating from fabrication. We show how to control the conversion of path to polarization qubits for sending MUBs, and conversely from polarization to path qubits when receiving them: we identify the so-called tilt angle at the conversion station as the parameter to control and improve the preparation of MUBs. We investigate the optimization of a tilt angle for various figures of merit, such as fidelities of MUBs, photon losses, and secret key rates in a practical realization. Our results suggest a realistic, cost-effective strategy for exploiting chip-based fiber-optic quantum communication.

DOI
10.1140/epjqt/s40507-026-00542-7

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In this work, we consider an on-chip silicon device that prepares photonic qubits in mutually unbiased bases (MUBs), which can be used to realize quantum key distribution protocols. We present a realistic approach to dealing with generic imperfections in on-chip devices for generating MUBs, in particular, imperfections originating from fabrication. We show how to control the conversion of path to polarization qubits for sending MUBs, and conversely from polarization to path qubits when receiving them: we identify the so-called tilt angle at the conversion station as the parameter to control and improve the preparation of MUBs. We investigate the optimization of a tilt angle for various figures of merit, such as fidelities of MUBs, photon losses, and secret key rates in a practical realization. Our results suggest a realistic, cost-effective strategy for exploiting chip-based fiber-optic quantum communication.
Silicon photonics holds the promise of the miniaturization of quantum communication devices. Recently, silicon chip optical transmitters for quantum key distribution (QKD) have been built and demonstrated experimentally. Nonetheless, these silicon chips suffer substantial phase- and polarization-dependent loss (PDL) which, if unchecked, could compromise the security of QKD systems due to overestimation of the secret key rate. Here, first, we restore the security by regarding single photons without phase and polarization dependence as untagged and secure qubits. Next, by using a postselection technique, we implement a secure QKD protocol that provides a high key generation rate even in the presence of severe phase- and polarization-dependent loss. Our solution is simple to realize in a practical experiment, as it does not require any hardware modification.
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-based polarization-encoding quantum key distribution (QKD) has been widely studied, owing to its low cost and robustness. However, prior studies have utilized off-chip devices to demodulate the quantum states or perform polarization compensation, given the difficulty of fabricating polarized independent components on the chip. In this paper, we propose a fully chip-based decoder for polarization-encoding QKD. The chip realizes a polarization state analyzer and compensates for the BB84 protocol without requiring additional hardware. It is based on a polarization-to-path conversion method that uses a polarization splitter-rotator. The chip was fabricated using a standard silicon photonics foundry; it has a compact design and is suitable for mass production. In the experimental stability test, an average quantum bit error rate of 0.56%0.56\%0.56% was achieved through continuous operation for 10 h without any polarization feedback. Furthermore, using the developed feedback algorithm, the chip enabled the automatic compensation of the fiber polarization drift, which was emulated by a random fiber polarization scrambler. In the case of the QKD demonstration, we obtained a finite-key secret rate of 240 bps over a fiber spool of 100 km. This study represents an important step toward the integrated, practical, and large-scale deployment of QKD systems.
Abstract Quantum communication has extremely high security and is a development direction of future communication networks. In this paper, an on-chip path, off-chip polarization encoded quantum key distribution (QKD) chip is designed and fabricated using advanced silicon photonics fabrication technology. Using a special two-dimensional grating, we can convert light of different path states into corresponding polarization states and propagate in fiber. The performance of the basic optics, waveguides, gratings, and Mach–Zehnder interferometers on the chip is characterized. In the demonstrated QKD experiment, the quantum bit error rate is as low as 1.08%. It provides an important device for the future development of advanced integrated quantum information.
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.
Silicon (Si) photonics is forming a fabless ecosystem, which is enabling low-cost and densely integrated components for optical communications and quantum information. We present a Si optical transmitter for polarization-encoded quantum key distribution (QKD). The chip was fabricated in a standard Si photonic foundry process and integrated together a pulse generator, intensity modulator, variable optical attenuator, and polarization modulator in a 1.3 mm×3 mm1.3 mm×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. This work shows the potential of using foundry Si photonics for low-cost, wafer-scale fabricated components for quantum information.