A modulator-free quantum key distribution transmitter chip

www.nature.com/npjqi ARTICLE OPEN A modulator-free quantum key distribution transmitter chip Taofiq K. Paraïso1, Innocenzo De Marco Zhiliang Yuan1 and Andrew J. Shields1 1,2 , Thomas Roger1, Davide G. Marangon1, James F. Dynes1, Marco Lucamarini 1 , Quantum key distribution (QKD) has convincingly been proven compatible with real life applications. Its wide-scale deployment in optical networks will benefit from an optical platform that allows miniature devices capable of encoding the necessarily complex signals at high rates and with low power consumption. While photonic integration is the ideal route toward miniaturisation, an efficient route to high-speed encoding of the quantum phase states on chip is still missing. Consequently, current devices rely on bulky and high power demanding phase modulation elements which hinder the sought-after scalability and energy efficiency. Here we exploit a novel approach to high-speed phase encoding and demonstrate a compact, scalable and power efficient integrated quantum transmitter. We encode cryptographic keys on-demand in high repetition rate pulse streams using injection-locking with deterministic phase control at the seed laser. We demonstrate record secure-key-rates under multi-protocol operation. Our modulator-free transmitters enable the development of high-bit rate quantum communications devices, which will be essential for the practical integration of quantum key distribution in high connectivity networks. npj Quantum Information (2019)5:42 ; https://doi.org/10.1038/s41534-019-0158-7 INTRODUCTION Information secrecy is an important challenge of modern society. Quantum cryptography,1 which aims at providing information theoretic security, is anticipated to be a major ingredient of future communication networks.2 The maturity and potential of this technology is illustrated by numerous achievements such as satellite to ground3 Quantum key distribution (QKD), few nodes quantum access networks,4 long distance links5 and novel high secret key capacity protocols.6,7 To translate these notable successes into effective adoption of the technology, high bandwidth devices compatible with large-scale deployment are yet to be developed.8 The bandwidth of quantum transmitters can be increased by multiplexing a large number of channels but this is in-scalable with bulk optics.9 Photonic integrated circuits (PICs), which combine multiple optical components onto a small semiconductor chip, are the best candidates to respond to this demand. Recent demonstrations of all-integrated indium phosphide QKD transmitters10 and high-speed silicon photonics QKD encoders11–13 have shown the advantage of integration in terms of stability and miniaturization of single channels. However the difficulty in realizing quantum state encoding in compact and power efficient circuits still hinders progress towards high-density integration and therefore large-scale deployment of the QKD technology. In QKD protocols, cryptographic keys are commonly encoded in the phase of attenuated laser pulses.1 At the core of the quantum transmitter, the quantum state encoding engine needs to be able to encode or randomize multiple phase states with deterministic phase values or with high entropy random numbers.14 All the existing QKD PICs achieve this function on-chip using high-speed interferometric modulation. This approach, also in use in classical communications,15 requires integrating multiple large footprint electro-optic modulation components, which operate at high powers and are vulnerable to chirp, residual amplitude modulation and electrical crosstalk.16 Moreover, because of the need of phase coherence between the pulses, gain-switching is avoided and the same technique is again utilized to generate pulses from continuous wave laser sources. An approach to on-chip phase encoding free of such modulators is highly desirable as it would increase the scaling capacity and reduce the power footprint of QKD systems at the same time. In this work, we present a QKD transmitter chip that exploits the direct phase modulation approach recently introduced in bulk optics transmitters.17 This approach combines gain-switching, injection locking and ultra-fast phase modulation of the seed-laser to generate chirp-free pulses and to realize multi-level phase encoding without the need of high-speed modulators. The lack of non-reciprocal components such as circulators or isolators in photonic integrated platforms forbids direct conversion of the bulk optics setup into PICs. On-chip realisation is enabled by the pulsed operation and an appropriate balance of the powers, which allow the suppression of reciprocal seeding effects from the slave laser, otherwise detrimental to deterministic phase encoding in the Master laser. Our QKD transmitters can be used for time-bin encoded protocols18,19 as well as distributed phase reference20,21 protocols. We achieve record secure key rates of 270 and 400 kbps at 20 dB attenuation (100 km in standard single-mode fibre) for the decoy state BB84 and distributed phase shift (DPS) protocols, respectively. Our implementation of phase encoding will also be beneficial to advanced coherent optical communications.22 By encoding multi-level optical modulation signals with up to eight distinct phase states at high signal integrity and low operation voltages, we demonstrate the potentialities of our new transmitter chips beyond QKD. 1 Toshiba Research Europe Ltd., Cambridge, UK and 2School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, UK Correspondence: Taofiq K. Paraïso (taofiq) Received: 16 October 2018 Accepted: 3 May 2019 Published in partnership with The University of New South Wales T.K. Paraïso et al. 2 Master Laser VOA b Slave Laser Master Slave Tx Intensity Fibre Out (i) Tx RF Amplitude a 2mm (ii) RF P-I-N Rx (iii) Key 6mm (iv) Time bins c Bias-Tee PIC Amp PC Alice AWG VOA Bias-Tee DFB DFB Rx APDs/ SNSPDs SPCM Amp 1234567890():,; PC Bob SMU TEC LAN Fig. 1 Description of the quantum transmitter chip. a Schematic diagram of the photonic intregrated circuit. b Principle of operation of the phase-encoded seeding. c Description of the experimental setup RESULTS Quantum key distribution chip A simple schematic of the quantum transmitter chip is shown in Fig. 1a. Only three main building blocks are required to generate pulse trains of phase-encoded photons: two cascaded highbandwidth distributed feedback (DFB) lasers and one optical attenuator between them. For the sake of flexibility, a thermally tuneable Mach-Zehnder interferometer (MZI) is used as a variable optical attenuator. Light is coupled out of the chip into a tapered lensed fibre using a spot-size converter. In order to drive the DFB lasers at high speed, we combine DC biases produced by precision DC current sources and radiofrequency (RF) signals from an arbitrary waveform generator (AWG). Figure 1b plots schematically the RF sig (...truncated)