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Applied Optics

Quantum state preparation of time-bin encoding based on SOI integrated chips

Hanming Yang, Chunxue Zhang, Pengwei Cui, Junchi Ma, Liyong Guo, Song Lih Huang, Jianguang Li, Jiashun Zhang, Yue Wang, Xiaojie Yin, Wei Chen, Yuanda Wu, Junming An

Published Aug 5, 2025• 1 citation
Publisher

Abstract

Quantum key distribution (QKD) ensures unconditional security for public key encryption by utilizing the principles of quantum mechanics. This study designs and fabricates a QKD chip based on the silicon-on-insulator (SOI) platform, employing a time-bin encoding scheme with decoy states. In the design, we integrate a slow thermo-optic phase modulator with a carrier-depletion modulator to ensure high fidelity of quantum states and high-speed encoding capabilities, enabling precise and flexible time-bin encoding. We achieve the encoding and decoding of four BB84 quantum states at a repetition rate of 100 MHz. In the experiment, the visibility of the interference fringes for the phase state |+⟩ is 93.66%, and for the phase state |−⟩, it is 92.36%. The extinction ratios for the time states |0⟩ and |1⟩ are 19.33 and 18.72 dB, respectively. The experimental results demonstrate that the chip has efficient quantum state preparation capability, providing significant support for the practical implementation of quantum key distribution technology. Additionally, we propose a new chip structure, to our knowledge, using SOI and Si 3 N 4 heterogeneous integration. The SOI waveguide is used for high-speed modulation encoding, while the delay line is composed of Si 3 N 4 waveguides. This structure is designed to address the stability issues of the chip caused by temperature variations.

DOI
10.1364/ao.560473

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Chinese Physics B•Dec 8, 2021•14 citations
Quantum key distribution (QKD) ensures unconditional security for public key encryption by utilizing the principles of quantum mechanics. This study designs and fabricates a QKD chip based on the silicon-on-insulator (SOI) platform, employing a time-bin encoding scheme with decoy states. In the design, we integrate a slow thermo-optic phase modulator with a carrier-depletion modulator to ensure high fidelity of quantum states and high-speed encoding capabilities, enabling precise and flexible time-bin encoding. We achieve the encoding and decoding of four BB84 quantum states at a repetition rate of 100 MHz. In the experiment, the visibility of the interference fringes for the phase state |+⟩ is 93.66%, and for the phase state |−⟩, it is 92.36%. The extinction ratios for the time states |0⟩ and |1⟩ are 19.33 and 18.72 dB, respectively. The experimental results demonstrate that the chip has efficient quantum state preparation capability, providing significant support for the practical implementation of quantum key distribution technology. Additionally, we propose a new chip structure, to our knowledge, using SOI and Si 3 N 4 heterogeneous integration. The SOI waveguide is used for high-speed modulation encoding, while the delay line is composed of Si 3 N 4 waveguides. This structure is designed to address the stability issues of the chip caused by temperature variations.
Quantum key distribution (QKD) relies on quantum state manipulation to achieve unconditionally secure key transmission. Polarization encoding, a mainstream QKD scheme, faces practical limitations due to the bulkiness and instability of discrete optical components. This work presents a polarization-encoding chip based on hybrid thin-film lithium niobate (TFLN) and silicon-on-insulator (SOI) integration, enabling dynamic control of four polarization states | R 〉, | A 〉, | L 〉, and | D 〉 with extinction ratios over 16.78 dB, using only a single thermal and electro-optic phase shifter in a compact architecture. Experimental results demonstrate that the chip achieves a secure key rate (SKR) of 328 kbps and a quantum bit error rate (QBER) of 1.96% at 100 MHz repetition rate in a 10 km fiber-based QKD system. Compared to conventional silicon-based carrier-depletion modulators (CDMs), the TFLN-SOI hybrid integration design avoids phase saturation and dynamic loss induced by carrier depletion, significantly enhancing the stability and scalability of polarization encoding.
We demonstrate a thin-film lithium niobate (TFLN) integrated photonic chip for time-binencoded quantum key distribution (QKD). The chip enables on-chip preparation of both time- and phase-basis quantum states required by the BB84 protocol. By exploiting the linear Pockels effect in TFLN, high-fidelity phase modulation is achieved without phase-dependent loss, resulting in stable and low-noise quantum state generation. The measured interference visibility reaches up to 98.74% for phase-basis states, while extinction ratios exceeding 22 dB are obtained for time-basis states. Furthermore, a chip-to-chip QKD system is implemented using TFLN integrated circuits at both the transmitter and the receiver. Finite-key analysis based on a one-decoy protocol demonstrates positive secret key generation over 20 km of standard single-mode fiber.
Abstract Quantum key distribution (QKD) is a method for secure communication that utilizes quantum mechanics principles to distribute cryptographic keys between parties. Integrated photonics offer benefits such as compactness, scalability, energy efficiency and the potential for extensive integration. We have achieved BB84 phase encoding and decoding, time-bin phase QKD, and the coherent one-way (COW) protocol on a planar lightwave circuit (PLC) platform. At the optimal temperature, our chip successfully prepared quantum states, performed decoding and calculated the secure key rate of the time-bin phase-decoding QKD to be 80.46 kbps over a 20 km transmission with a quantum bit error rate (QBER) of 4.23%. The secure key rate of the COW protocol was 18.18 kbps, with a phase error rate of 3.627% and a time error rate of 0.377%. The uniqueness of this technology lies in its combination of high integration and protocol flexibility, providing an innovative solution for the development of future quantum communication networks.
The realization of large-scale, practical quantum key distribution (QKD) networks relies on the development of scalable architectures and integrated hardware. We demonstrate a metropolitan wavelength-division multiplexing (WDM) QKD network based on hybrid-integrated quantum node transceiver chips that integrate both transmitter and receiver functionalities, including high-speed pulse modulation, decoy-state modulation, tunable attenuation, time-bin encoding, and decoding. The packaged module offers a wavelength tuning range exceeding 85 nm, with its silicon photonic Mach–Zehnder modulator (MZM) achieving a single-photon equivalent dynamic extinction ratio (SP-EDER) of over 20 dB across the entire tuning range at a 1.25 GHz pulse repetition rate. In combination with a simple 1.25 GHz clocked integrated time-bin QKD system, we generate secret keys at a rate of 865.4 kbps over a channel with 9.08 dB of fixed attenuation and at a rate of 168.2 kbps over a distance of 80 km at 1550.12 nm. Our work demonstrates an integrated WDM QKD network capable of supporting scalable and cost-effective metropolitan quantum-secure communications.
Integrated photonics provides a promising platform for quantum key distribution (QKD) system in terms of miniaturization, robustness, and scalability. Tremendous QKD works based on integrated photonics have been reported. Nonetheless, most current chip-based QKD implementations require additional off-chip hardware to demodulate quantum states or perform auxiliary tasks such as time synchronization and polarization basis tracking. Here, we report a demonstration of resource-efficient chip-based BB84 QKD with a silicon-based encoder and a decoder. In our scheme, the time synchronization and polarization compensation are implemented relying on the preparation and measurement of the quantum states generated by on-chip devices; thus, we need no additional hardware. The experimental tests show that our scheme is highly stable with a low intrinsic quantum bit error rate of 0.50 % ±0.02 % in a 6 h continuous run. Furthermore, over a commercial fiber channel up to 150 km, the system enables the realization of secure key distribution at a rate of 866 bit/s. Our demonstration paves the way for a low-cost, wafer-scale manufactured QKD system.
A quantum key distribution transmitter chip based on hybrid-integration of silica planar light-wave circuit (PLC) and lithium niobates (LN) modulator PLC is presented. The silica part consists of a tunable directional coupler and 400-ps delay line, and the LN part is made up of a Y-branch, with electro-optic modulators on both arms. The two parts are facet-coupled to form an asymmetric Mach–Zehnder interferometer. We successfully encode and decode four BB84 states at 156.25-MHz repetition rate. Fast phase-encoding of 0 or π is achieved, with interference fringe visibilities 78.53% and 82.68% for states |+〉 and |–〉, respectively. With the aid of an extra off-chip LN intensity modulator, two time-bin states are prepared and the extinction ratios are 18.65 dB and 15.46 dB for states |0〉 and |1〉, respectively.