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An InP-based integrated modulated coherent state source for differential phase shift quantum key distribution

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

Published Jan 1, 2019• 2 citations
OpenAlex

Abstract

A low-cost 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 is presented. Initial characterization results of the laser, and the modulator sections are demonstrated.

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This paper presents a quantum key distribution (QKD) protocol named differential-quadrature-phase-shift (DQPS) QKD. A transmitter sends a coherent pulse train in which each pulse is phase modulated by ${0,\ensuremath{\pi}}$ ${\ensuremath{\pi}/2,3\ensuremath{\pi}/2}$, and a receiver measures it with either one of two asymmetric Mach-Zehnder interferometers having phase biases of 0 and $\ensuremath{\pi}/2$, respectively. This scheme is devised such that the idea of the conventional Bennett-Brassard 1984 protocol (BB84) is introduced to differential-phase-shift (DPS) QKD. Simulations show that the proposed scheme is robust against a sequential attack that is crucial eavesdropping to DPS-QKD.
Developing quantum key distribution (QKD) systems using monolithic photonic integrated circuits (PICs) can accelerate their adoption by a wide range of markets, thanks to the potential reduction in size, complexity of the overall system, power consumption, and production cost. In this work, we design, fabricate and characterize an InP-based PIC transmitter for continuous-variable (CV) QKD applications. In a proof-of-principle experiment implementing a pulsed Gaussian-modulated coherent state (GMCS) CV-QKD protocol over an optical fiber channel of 11 km, the system showed a performance compatible with a secret key rate of 78 kbps in the asymptotic regime. These results show the potential of InP technologies to integrate CV-QKD systems onto a monolithic platform.
Quantum key distribution (QKD) allows secret key exchange between two users with unconditional security. For QKD to be widely deployed, low cost and compactness are crucial requirements alongside high performance. Currently, the majority of QKD systems demonstrated rely on bulk intensity and phase modulators to generate optical pulses with precisely defined amplitude and relative phase difference—i.e., to encode information as signal states and decoy states. However, these modulators are expensive and bulky, thereby limiting the compactness of QKD systems. Here, we present and experimentally demonstrate a novel optical transmitter design to overcome this disadvantage by generating intensity- and phase-tunable pulses at GHz clock speeds. Our design removes the need for bulk modulators by employing directly modulated lasers in combination with optical injection locking and coherent interference. This scheme is, therefore, well suited to miniaturization and photonic integration, and we implement a proof-of-principle QKD demonstration to highlight potential applications.