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One photon, one click

Especially interesting for
  • quantum information technology
  • medicine
  • biotechnology
  • astronomy

Today, single-photon sources and detectors are not used solely in experimental quantum physics, but also, for example, in medicine, biotechnology and astronomy. They play a particular role in quantum information technology (QIT) (quantum computers, quantum cryptography) which is to ensure secure data interchange in future. Due to the large number of possible applications, the need for calibrations and validations of the measuring instruments and measurement techniques required is increasing. At PTB, calibrations of single-photon detectors have been carried out for the fi rst time with nonclassical light.

Confocal image of nano-diamonds. The light areas show the emissions of the centres of nitrogen discontinuities which are used as single-photon sources. Top right: The measurement of the second-order correlation function shows a drop at t = 0, which is a clear indicator of a single-photon source. Bottom right: Infl uence of the photon statistics of different radiation sources and of the mean number of photons on the calibration result of two singlephoton detectors.

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 Kück
Department 4.5
Optical Technologies
phone: +49 (0) 531 592-4500
e-mail: stefan.kueck(at)ptb.de

Scientific publication

Schmunk, W.; Rodenberger, M.; Peters, S.; Hofer, H.; Kück, S.: Radiometric calibration of single photon detectors by a single photon source based on NV-centers in diamond. Journal of Modern Optics 58 (2011) 1252 – 1259