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Because every photon counts

Tommaso Lunghi

à 11h en salle C. brot

A range of applications always more broad requires the efficient generation, manipulation and detection of single photons. In some cases, e.g. in medical applications, the goal consists only in the detection of very faint light. In other cases, e.g. in Quantum Key Distribution, the quantum nature of light is exploited. On that basis, innovative devices are being developed. Only recently, research institutes are trying to achieve an agreement on the relevant features to be characterized and how to characterize them. Recently a step further has been done and several protocols to characterize the efficiency of a single-photon detector have been cross-validated by a consortium of metrological institutes (the candela project). However, the diffusion of these protocols is prevented by the significant lack of precision in the measurement of the optical power at low level. Absolute measurement can only be obtained thanks to high-skilled competences and expensive technologies, which can rarely be found outside a metrology laboratory. Another important challenge in quantum-information applications consists in the estimation of the entropy, i.e. the randomness, of the data generated by a given device. Single photons can be used to generate completely unpredictable outcomes but, in practical realization, the quantum process is mixed with classical imperfections that might be controlled to produce predictable outcomes. To prevent this, the device must be carefully modelled together with all its imperfections. I’m going to present a possible solution to these two problems : In the first part of my talk I will show how to realize a radiometer able to measure absolutely, i.e. without relying on any previous calibrations, the power of a low intensity beam. This radiometer is based on an Erbium-doped fiber and other standard optical-fiber components, therefore can be used in any laboratory as a primary measurement standard of optical radiance. Thanks to this device, we characterized a single-photon detector with precision smaller than 1%. In the second part of my talk I will show a protocol to generate random bit with certified entropy. This quantum random number generator is self-testing, i.e. that the user can monitor the entropy in real-time. Based on a few general assumptions, our protocol guarantees continuous generation of high quality randomness, without needing of a detailed characterization of the devices. Using a fully optical setup, we implement this protocol and illustrate its self-testing capacity.

References :

[1] Metrology of single-photon sources and detectors : a review C. J. Chunnilall ; I. P. Degiovanni ; S. Kück ; I. Müller ; A. G. Sinclair Opt. Eng. 53(8), 081910 (2014)

[2] Absolute calibration of fiber-coupled single-photon detector T. Lunghi, B. Korzh, B. Sanguinetti, H. Zbinden Opt. Exp. 22(15) 18078-18092 (2014)

[3] A self-testing quantum random number generator T. Lunghi, J. B. Brask, C. C. W. Lim, Q. Lavigne, J. Bowles, A. Martin, H. Zbinden, N. Brunner arXiv:1410.2790 (2014)