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IEEE photonics journal

Quantum State Preparation for Quantum Key Distribution Using PLC Module

Dan Wu, Shaokang Chen, Pengwei Cui, Junchi Ma, Wei Chen, Jiashun Zhang, Yue Wang, Jianguang Li, Junming An

Published Oct 5, 2023• 9 citations

Abstract

Quantum Key Distribution (QKD) is gaining significant interest due to its theoretical guarantee of unconditional security. Integrated optics offers distinct advantages compared to bulk optics, including high stability, flexibility, controllability, and robustness. We develop a planar lightwave circuit (PLC) silica asymmetric Mach-Zehnder interferometer (AMZI) module with temperature system at an accuracy of 0.1 ℃ including an AMZI chip with a 400ps delay and a single-chip microcomputer. The silica AMZI module is used to encode at Alice, and the same low loss AMZI module is used to decode at Bob at a clock repetition rate of 156 MHz. Two intensity states Z -basis (|0ñ, |1ñ) and two phase states X-basis (|+ñ, |-ñ) are prepared at Alice and detected at Bob successfully. The extinction ratio of |0ñ and |1ñ between first-slot and third-slot are about 17 dB. And the interference visibility of |+ñ, |-ñ between interference max and min of second-slot are about 88%. The estimated secret key rate and the quantum bit error rate (QBER) using a simulation model for a transmission distance of 20 km is 16.3 kbps and 0.0384, respectively.

DOI: 10.1109/jphot.2023.3322150

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Silica-based planar lightwave circuit (PLC) devices can offer great potential for quantum key distribution (QKD) with the benefits of low-loss, low-cost, large-scale integration, miniaturization, stability and mass production. A quantum decoding integrated chip for QKD based on silica PLC technology was demonstrated, which consists of a variable optical splitter (VOS), an asymmetric Mach–Zehnder interferometer (AMZI) with a thermo-optic phase modulator (TOPM) and a delay line (DL) whose delay time is 400ps, and a variable directional coupler (VDC). The balanced pulsepairs of double channels with measured delay times of 396ps and 398ps respectively were obtained in the conditions of both classical optics and single-photon transmissions. The chip has advantageous to achieve high interference visibilities of double channels simultaneously for BB84 phase and time-bin protocols.

Quantum key distribution (QKD) provides a solution for communication of unconditional security. However, the quantum channel disturbance in the field severely increases the quantum bit-error rate, degrading the performance of a QKD system. Here we present a setup comprising silica planar light wave circuits (PLCs), which is robust against the channel polarization disturbance. Our PLCs are based on the asymmetric Mach–Zehnder interferometer (AMZI), integrated with a tunable power splitter and thermo-optic phase modulators. The polarization characteristics of the AMZI PLC are investigated by a novel pulse self-interfering method to determine the operation temperature of implementing polarization insensitivity. Over a 20 km fiber channel with 30 Hz polarization scrambling, our time-bin phase-encoding QKD setup is characterized with an interference fringe visibility of 98.72%. The extinction ratio for the phase states is kept between 18 and 21 dB for 6 h without active phase correction.

Silica-based planar lightwave circuit (PLC) devices can reduce transmission loss and cost in a quantum key distribution (QKD) system, and have potential applications in integration and production. A PLC-based quantum decoding integrated chip for multi-protocols is designed and fabricated, which is composed of variable optical splitters (VOSs), asymmetric Mach–Zehnder interferometers (AMZIs), and variable directional couplers (VDCs). Balanced pulse-pairs of four outputs are obtained simultaneously with measured delay times of 405 ps and 402 ps, respectively. The chip has advantages in achieving high interference visibility and low quantum bit error rate (QBER).

Quantum key distribution (QKD) system based on passive silica planar lightwave circuit (PLC) asymmetric Mach–Zehnder interferometers (AMZI) is characterized with thermal stability, low loss and sufficient integration scalability. However, waveguide stresses, both intrinsic and temperature-induced stresses, have significant impacts on the stable operation of the system. We have designed silica AMZI chips of 400 ps delay, with bend waveguides length equalized for both long and short arms to balance the stresses thereof. The temperature characteristics of the silica PLC AMZI chip are studied. The interference visibility at the single photon level is kept higher than 95% over a wide temperature range of 12 °C. The delay time change is 0.321 ps within a temperature change of 40 °C. The spectral shift is 0.0011~nm/0.1 °C. Temperature-induced delay time and peak wavelength variations do not affect the interference visibility. The experiment results demonstrate the advantage of being tolerant to chip temperature fluctuations.

Many improvements are required to bring quantum key distribution (QKD) from the laboratory to the commercial market, such as simplifying structure and enhancing stability. In this letter, a QKD module based on planar lightwave circuit (PLC) has been proposed. The chip inside module is based on silica-silicon material and composed of various optical splitter, asymmetric Mach–Zehnder interferometer and directional coupler. The module can allow users adjust temperature stably. The interference visibility can be high to 96% merely by controlling and stabilizing temperature and voltage of the module through self-interfering method which showed its potential in practical application.

Quantum key distribution can realize unconditional security of communication. Here, a quantum decoding chip based on silica-on-silicon planar lightwave circuit (PLC) platform, which is compatible for multiprotocol has been demonstrated. PLC technology provides stability, high integration, and practicality for the chip, whereas silica-on-silicon material makes it low loss and low cost. The chip is integrated with three variable optical splitters, two asymmetric Mach–Zehnder interferometers, and four variable directional couplers. The interference visibility is investigated by self-interfering method, and the results showed it is high to 98.2% under temperature control. The extinction ratio for the phase states is kept between 23 and 25 dB for 6 h without active phase correction.