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Monolithically integrated differential phase shift transmitter for quantum key distribution

Ozan Çirkinoglu, Rui Santos, Kevin A. Williams, Xaveer J. M. Leijtens

Published Jun 24, 2020• 2 citations
OpenAlex

Abstract

An integrated differential phase shift QKD transmitter, which consist of a distributed Bragg reflector laser, a phase modulator, and Mach-Zehnder modulator sections for pulse carving and optical attenuation has been designed and fabricated. Initial characterization shows single mode operation of the laser with a linewidth around 2 MHz, and an achievable optical attenuation of more than 60 dB.

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An integrated differential phase shift QKD transmitter, which consist of a distributed Bragg reflector laser, a phase modulator, and Mach-Zehnder modulator sections for pulse carving and optical attenuation has been designed and fabricated. Initial characterization shows single mode operation of the laser with a linewidth around 2 MHz, and an achievable optical attenuation of more than 60 dB.
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We demonstrate the first fully integrated continuous-variable quantum key distribution (CV-QKD) transmitter, combining a narrow-linewidth laser, high-speed IQ modulator, and variable optical attenuator on a single indium phosphide (InP) chip. The transmitter occupies a compact footprint of 2 mm by 4.6 mm. Component characterization confirms single-mode laser operation with 120 kHz linewidth, >8 GHz modulator bandwidths, and 58 dB attenuation range. Deployed in a complete heterodyne CV-QKD system with probabilistically shaped 64-QAM, the system achieves an average excess noise of 0.1 shot noise units (SNU) and a secret key rate of 8.4 Mbit/s over a 14.3 km fiber link in the asymptotic regime. These results establish a practical and scalable route toward fully integrated quantum-secure communication systems.
For the establishment of secure communication as well as a future quantum internet, technologies like Quantum Key Distribution (QKD) must become smaller in size, cost-effective in production and more performant. Since attacks like store-now-decrypt-later are already a threat for sensitive data, research in this area must be pushed towards a higher practicality. In this work we show the feasibility of using a photonic integrated circuit (PIC) for application as a QKD transmitter. The PIC (provided by Eindhoven University of Technology) comprises a ring laser source, a Mach-Zehnder interferometer (MZI) with integrated RF phase modulators used for pulse carving and phase modulation as well as an attenuator section. This indium phosphide (InP) chip enables us to generate the necessary states for a 4-state time-bin-phase BB84 protocol [1] implementation. After characterization of all the photonic components and preliminary testing, we were able to generate a secret key rate of approximately 89kbps for 50MHz signal repetition rate with a QBER of 1.3% (in Z-Basis) in a back-to-back configuration.