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Single-photon nanostructured detectors for advanced optical applications

Final Report Summary - SINPHONIA (Single-photon nanostructured detectors for advanced optical applications)

The goal of 'Single-photon nanostructured detectors for advanced optical applications' (Sinphonia) was to develop and investigate a specific type of single-photon detector based on superconductor nanostructures, and demonstrate its use in a number of applications requiring ultimate sensitivity in the near-infrared (IR) and high speed of operation. These superconducting single-photon detectors (SSPDs), demonstrated for the first time by one of Sinphonia's partners, rely on the formation of a resistive 'hot spot' in a superconducting nanostripe upon absorption of a single photon, and on the consequent generation of a voltage pulse. Dark count rates as small as 1 counts / s can be achieved, several orders of magnitudes lower than semiconductor detectors with comparable efficiency in the near-IR. Unlike other approaches to single-photon detection using superconductors, the localised nature of the hot spot allows an ultrafast (˜20-30 ps) thermalisation of the excitation and therefore a response time several orders of magnitudes faster than e.g. avalanche photodiodes.

Sinphonia has pushed the technology of ultrathin superconducting films much beyond the state-of the- art. SSPDs have been integrated into fibre-coupled, user-friendly systems including liquid-He or cryogen free cooling, bias and amplification electronics. In particular, two generations of pulse-tube coolers, specifically designed for SSPDs, have been developed. These systems have been extensively used in quantum cryptography (QC) applications, enabling the demonstration of several record-breaking experiments. Entanglement swapping using continuous-wave sources, quantum key distribution over 150 km of installed fibre and over 250 km of fibre-on-a-spool, were demonstrated with SSPDs, setting the state-of-the-art for long-distance quantum communications. Additionally, SSPDs were successfully integrated in a time-coding QC set-up and a commercial plug and play QC appliance, showing the maturity and flexibility of the technology. Optical data transmission at 2.5 Gb / s was also demonstrated using fast SSPDs operated in linear mode, as a first step towards application in ultralong-distance optical communications.

Overall, the Sinphonia consortium defined the state-of-the-art for the device performance in terms of sensitivity (QE / dark count rate) and speed, for the device functionality (photon-number-resolution, pre-amplification) and for applications (best available fibre-coupled SSPD system, record distances in QKD). The consortium's activity has been very visible in conference and journal publications, including papers in top journals (Nature Physics and Nature Photonics). Few other groups worldwide (MIT and Jet Propulsion Laboratory in the USA, NICT in Japan and TU Delft in Europe) have demonstrated an independent fabrication process of high-efficiency SSPDs. In terms of efficiency, the world record (57 % at 1550 nm) has been reported at MIT using integrated top mirrors, but the dark count rate was not reported. In terms of applications, the use of SSPDs fabricated at MSPU has been demonstrated by other groups (NIST, Stanford) in quantum optics and quantum communication experiments, most notably for long-distance QKD (200 km in the laboratory). The development of Sinphonia cryogen-free system has recently provided the consortium with a more sensitive fibre-coupled detector, allowing us to establish a new record of 250 km, and to demonstrate a field-trial over 150 km of installed fibre.

Sinphonia has reached the large majority of its objectives. Among the 23 technical milestones over the entire project duration, 19 were achieved, one was partially achieved, and only three were not achieved (due to technical problems). In many cases (photon-number-resolution, pre-amplification, high speed) the milestones have been achieved through innovative technical solutions totally unexpected at the beginning of the project, showing the innovative and dynamic character of Sinphonia work. Most importantly, Sinphonia has fully achieved its main objectives:

- to fabricate single-photon optical detectors with unprecedented performance at telecom wavelengths;
- to demonstrate their implementation in several IST applications by industrial partners and by doing that has developed and promoted the SSPD technology to the point where it has become a key technology for single-photon applications.

Sinphonia has been extremely active in disseminating the scientific results of the projects activity, as well as promoting the use of the SSPD technology. 29 papers have been submitted / published in peer-reviewed journals, and 55 papers have been presented at international conferences, including many invited papers. A special symposium on SSPDs has been organised in the framework of the Single-Photon Workshop SPW2007, with a strong representation of both Sinphonia partners and external laboratories. Sinphonia papers have been presented in all major events on superconductive devices and single-photon detectors. Additionally, the SSPD technology has been promoted by invited seminars in external laboratories and on-site lab demonstrations. Extended information on the project is available on the public part of the project’s website: www.sinphonia.org. On the exploitation side, five project-related patents have been filed by Sinphonia partners during the projects lifetime, and patent searches and market analyses have been periodically carried out. Use and dissemination plans have been produced both at the mid-term and at project end.

During the project lifetime, and in large part due to the partners efforts, SSPDs have evolved from a technological curiosity to an established technology, widely recognised as the key approach to ultrasensitive single-photon measurements. The Sinphonia consortium has identified different areas of applications where SSPDs can find use. On one hand, the optical instrumentation market represents already today an interesting, small-volume market for SSPDs. On the other hand, quantum key distribution, remote sensing, picosecond integrated circuit analysis and optical communication can open up larger markets in the medium term.

A first commercial solution is already available from a spin-off of a Sinphonia partner, and has found initial acceptance in the instrumentation market. Future plans include the development of Sinphonia's technical breakthroughs as commercial products, and extending the market share by the development of more advanced system solutions including cryogen-free cooling. The vigorous research activities deployed during the project will continue in Sinphonia and other laboratories and will contribute to the further development of this exciting research and application field.

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