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.
We experimentally demonstrate a cost-effective coherent 10 Gb/s system for passive optical networks, exploiting off-the-shelf DFB lasers and a phase-diversity receiver based on a simple 3 & times; 3 fiber coupler. Since the system uses a simple amplitude-shift keying format, no complex electronic processing is required and there is no need of frequency/phase stabilization of the local oscillator, whose frequency can change by more than & plusmn;1 GHz with no significant performance variation. The system has a 40 dB loss budget and is, therefore, compatible with the high losses of practical optical distribution networks, where power splitting is used to distribute the signal to a high number of users. Error-free 10-Gb/s transmission at the FEC limit is obtained after transmission over up to 66 km of G.652 single mode fiber. Polarization-independent operation is also demonstrated with a simple modification of the detection scheme, without duplicating components, and with a small variation of the sensitivity. The limited complexity indicates the potential for a cost-effective implementation, which makes it compatible with the strictly cost-Aware access networks environment, even for high-end services
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