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Journal of Modern Optics

Quantum encoder and decoder for practical quantum key distribution using a planar lightwave circuit

Yoshihiro Nambu, K. Yoshino, Akihisa Tomita

Published Jul 2, 2008• 58 citations
Publisher

Abstract

To launch quantum key distribution (QKD) into the commercial market, it is important to develop a system that is simpler and more reliable using current technology. This report proposes quantum encoders and decoders using a passive planar lightwave circuit (PLC) that is useful for implementing optical-fiber-based QKD systems. Our encoders and decoders are based on an asymmetric Mach–Zehnder interferometer and allow us to prepare and analyze various photonic time-bin qubits reliably. The system can be stable and polarization-insensitive merely by stabilizing and controlling the device temperature. Our PLC-based devices enables us to simplify the QKD system and increase its reliability.

DOI
10.1080/09500340801942414

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To launch quantum key distribution (QKD) into the commercial market, it is important to develop a system that is simpler and more reliable using current technology. This report proposes quantum encoders and decoders using a passive planar lightwave circuit (PLC) that is useful for implementing optical-fiber-based QKD systems. Our encoders and decoders are based on an asymmetric Mach–Zehnder interferometer and allow us to prepare and analyze various photonic time-bin qubits reliably. The system can be stable and polarization-insensitive merely by stabilizing and controlling the device temperature. Our PLC-based devices enables us to simplify the QKD system and increase its reliability.
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.
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) 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.
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).
We developed a one-way quantum key distribution (QKD) system based upon a planar lightwave circuit (PLC) interferometer. This interferometer is expected to be free from the backscattering inherent in commercially available two-way QKD systems and phase drift without active compensation. A key distribution experiment with spools of standard telecom fiber showed that the bit error rate was as low as 6% for a 100-km key distribution using an attenuated laser pulse with a mean photon number of 0.1 and was determined solely by the detector noise. This clearly demonstrates the advantages of our PLC-based one-way QKD system over two-way QKD systems for long distance key distribution.
We developed a one-way quantum key distribution (QKD) system based upon a planar lightwave circuit (PLC) interferometer. This interferometer is expected to be free from the backscattering inherent in commercially available two-way QKD systems and phase drift without active compensation. A key distribution experiment with spools of standard telecom fiber showed that the bit error rate was as low as 6% for a 100-km key distribution using an attenuated laser pulse with a mean photon number of 0.1 and was determined solely by the detector noise. This clearly demonstrates the advantages of our PLC-based one-way QKD system over two-way QKD systems for long distance key distribution.