Single-photon sources are nonclassical light sources which emit one single photon per time interval. Within the scope of the projects qu-Candela (www.quantumcandela. org) and EPHQUAM (www. ephquam.de), single-photon sources based on the laser-induced emission of the nitrogen vacancy centres in nanocrystalline diamond have, among other things, been investigated. These light sources emit single photons; up to 106 such photons per second can be detected experimentally.
The emission of a single photon is proven by means of correlation measurements with a beam splitter: a single-photon detector is positioned on each of the beam splitter's outputs (Hanbury-Brown- Twiss interferometer). If single photons hit the beam splitter, they can head to either of the detectors. The probability that both detectors "click" simultaneously is, thus, zero. Hence, single-photon sources are ideal for the calibration of single-photon detectors (so-called "click detectors"), since they – contrary to classical light optical receivers – register only photons which incide singly. If, however, classical light sources are used, several photons reach the single-photon detector simultaneously; the consequence will be calibration errors. This can be demonstrated by means of numerical modelling: already at photon fl uxes at which 0.1 photons on average hit the single-photon detector per detection time interval, errors > 1.2 % (laser) or > 2.5 % (thermal light) of the measured detection effi ciency occur, whereby the measurement uncertainty of the investigations carried out at the moment lies at around < 1 %.
In future, spectral infl uences are to be investigated in detail for the determination of the relative and of the absolute detection effi ciency of single-photon detectors; also alternative methods are to be used.
stefan.kueck