Full daylight quantum-key-distribution at 1550 nm enabled by integrated silicon photonics

www.nature.com/npjqi ARTICLE OPEN Full daylight quantum-key-distribution at 1550 nm enabled by integrated silicon photonics 1234567890():,; M. Avesani 1,8, L. Calderaro 1,8, M. Schiavon1,6,8, A. Stanco 1,8, C. Agnesi 1, A. Santamato1,7, M. Zahidy1, A. Scriminich1, G. Foletto1, G. Contestabile2, M. Chiesa3, D. Rotta3, M. Artiglia4, A. Montanaro4, M. Romagnoli 4, V. Sorianello 4, F. Vedovato 1,8, G. Vallone 1,5 and P. Villoresi 1 ✉ The future envisaged global-scale quantum-communication network will comprise various nodes interconnected via optical fibers or free-space channels, depending on the link distance. The free-space segment of such a network should guarantee certain key requirements, such as daytime operation and the compatibility with the complementary telecom-based fiber infrastructure. In addition, space-to-ground links will require the capability of designing light and compact quantum devices to be placed in orbit. For these reasons, investigating available solutions matching all the above requirements is still necessary. Here we present a full prototype for daylight quantum key distribution at 1550 nm exploiting an integrated silicon-photonics chip as state encoder. We tested our prototype in the urban area of Padua (Italy) over a 145 m-long free-space link, obtaining a quantum bit error rate around 0.5% and an averaged secret key rate of 30 kbps during a whole sunny day (from 11:00 to 20:00). The developed chip represents a cost-effective solution for portable free-space transmitters and a promising resource to design quantum optical payloads for future satellite missions. npj Quantum Information (2021)7:93 ; https://doi.org/10.1038/s41534-021-00421-2 INTRODUCTION Quantum Key Distribution (QKD)1–4 is the most advanced application of quantum information science, continuously improving in terms of new protocols5,6 and experimental realizations7–10. The potential of QKD is to allow secure communication between any two points on Earth. Depending on the link distance, the quantum channel is established through fiber-based or free-space quantum communication (QC). Despite the recent demonstrations also realized in satellite-to-ground links11–14, free-space QKDtechnology is currently limited and cannot compete with its fiberbased counterpart7,15–18. Hence, in the vision of a continentalscale quantum network (or quantum internet)19–23 where both types of link are required to jointly operate, certain key requirements for free-space QC can be formulated, as (i) full-day functionality, (ii) compatibility with standard fiber-based technology at telecom wavelength, and (iii) the achievement of stable coupling of the free-space signal into a single-mode fiber (SMF). Regarding (i), the background noise due to sunlight poses a serious limitation on the achievable performance of day-time freespace QC, limiting most of the demonstrations obtained so far to night-time. For this reason, various studies have focused on the feasibility of daylight QKD24–29. Most of them exploited light in the 700–900 nm band, which allows for a good atmospheric transmission, and to exploit commercial low-noise silicon-based single-photon avalanche diodes (SPADs). To reduce the background noise due to the Sun and to maintain, at the same time, a good efficiency in the atmospheric transmission, the choice to use light signals in the telecom C-band (around 1550 nm) has only very recently started to be investigated28,29. Moving the operating wavelength to the telecom band comes with (at least) two advantages. Firstly, given the availability of commercial-off-the-shelf components, using a working wavelength in the C-band is the standard choice in fiber-based optical (classical and) QC realizations, hence fulfilling the requirement (ii). This opens up the possibility of realizing a hybrid free-space to fiber system. For instance, it is particular important in a scenario where the ground station, collecting the photons from a satellite, is separated from the location of the detection stage. Secondly, it is compatible with integrated silicon photonics30–33, which represents a promising choice for designing light, compact, scalable and low power-consuming devices suitable for portable QKD transmitters and to design satellite optical payloads34,35. Furthermore, to match the requirement iii) it is necessary to actively compensate for the optical aberrations (at least the beam wander and angle-of-arrival fluctuation) introduced by atmospheric turbulence, which is experimentally challenging36,37. However, a stable coupling of the light signal into a SMF has the advantage of allowing the use of commercially available superconductive nanowire single-photon detectors (SNSPDs), which represent the standard for fiber-based state-of-the-art QKD demonstrations7,10,16. Here we address the requirements outlined above presenting a QC-system named “QCoSOne” (acronym for “Quantum Communication for Space-One”), which realizes free-space daylight QKD at 1550 nm. We exploited integrated silicon-photonics technology to realize a portable state encoder with decoy- and polarizationmodulation on a single chip, to implement the 3-state 1-decoy QKD protocol introduced in ref. 5. Moreover, the integrated QKD encoder has been put in a rugged package, designed and realized 1 Dipartimento di Ingegneria dell’Informazione, Università di Padova, via Gradenigo 6/B, Padova 35131, Italy. 2Istituto TeCIP - Scuola Superiore Sant’Anna, Pisa, Italy. 3InPhoTec, Integrated Photonic Technologies Foundation, Pisa, Italy. 4PNTLab - Consorzio Nazionale Interuniversitario per le Telecomunicazioni, Pisa, Italy. 5Dipartimento di Fisica e Astronomia, Università di Padova, Padova, Italy. 6Present address: Sorbonne Université, CNRS, LIP6, Paris, France. 7Present address: PNTLab - Consorzio Nazionale Interuniversitario per le Telecomunicazioni, Pisa, Italy. 8These authors contributed equally: M. Avesani, L. Calderaro, M. Schiavon, A. Stanco, F. Vedovato. ✉email: paolo. Published in partnership with The University of New South Wales M. Avesani et al. 2 1234567890():,; Fig. 1 Location of the field trial and QCoSOne setup. a Location of the field test in Padua. Map Data from Google [©2019 Google]. b Alice’s terminal with the sketch of the QKD source and chip schematic. DM dichroic mirror, CAM camera. c Bob’s terminal with the scheme of the state analyzer. d Picture of the packaged chip soldered to the external control board with the input/output fiber array. to the purpose in-house, making it ready for use in the field. We exploited commercially available wavelength filters (with ≲ 1 nm of bandwidth) and SNSPDs at the detection state. We reached a stable SMF coupling of the qubit stream over a 145m-long freespace link (Fig. 1a) by means of an active correction of the first order aberrations that allowed us to successfully perform QKD in daylight continuously from 11:00 to 20:00 (Central European Summer Time, CEST). We measured a quantum bit error rate (QBER) (...truncated)