metrologia Absolute detector quantum-efficiency measurements using correlated photons A. L. Migdall, R. U. Datla, A. Sergienko, J. S. Orszak and Y. H. Shih Abstract. An experimental system using correlated photons for radiometric purposes has been set up at the National Institute of Standards and Technology (NIST). We use pairs of correlated photons to produce spatial maps of the absolute efficiency of a photomultiplier photocathode at four wavelengths. We also compare this technique with measurements carried out by conventional means tied to existing radiometric standards. These initial comparisons show an average agreement within 0,6 % between the two methods. The results show that correlated photons can be a useful tool in radiometry and that the method holds enough promise to warrant further studies. 1. Introduction Correlated photons have been shown to hold promise in several areas of radiometric measurements [1, 2]. While several groups have demonstrated that cor- related pairs of photons can be used to determine the absolute quantum efficiency of photon-counting detectors without reference to any externally calibrated detector standard [3-8], no high-accuracy independent verification of the method has yet been published. In this work, we compare measurements of detector efficiency carried out by the correlated-photon technique with those carried out by conventional means tied to existing radiometric standards. The correlated-photon method is based on the process of parametric down-conversion, which produces optical photons in pairs within a nonlinear crystal. Within the crystal, photons from a pump laser beam, in effect “decay” into pairs of photons under the restrictions of energy and momentum conservation. Since the photons are created in pairs, the detection of one photon indicates the existence of the other. Because of the energy and momentum conservation requirements, the direction and energy of the detected photon can be used to predict not only the existence, but also the direction and energy of the other photon of the pair. The fact that one photon indicates the existence of a second photon allows absolute detector efficiencies to be determined. A. L. Migdall and R. U. Datla: Radiometric Physics Division, 221/B208, National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899, USA. A. Sergienko, J. S. Orszak and Y. H. Shih: Department of Physics, University of Maryland, Baltimore County, Baltimore, MD 21228, USA. To test the method, we have set up a system using correlated photons to map the spatial variations of the absolute quantum efficiency of a photomultiplier (PMT) photocathode at four wavelengths. To our knowledge, this is the first time that this method has been used to provide detailed spatial-response maps consisting of direct absolute measurements at each spatial position, rather than a relative-response map which is tied to an absolute scale at only one or a few points. To independently verify the measurement results, a simultaneous measurement of efficiency was made that was tied to an absolute radiometric scale [9]. These initial comparisons are useful to reveal any previously unconsidered systematic effects. 2. Experiment 2.1 Spatial measurement of absolute PMT efficiency To produce correlated pairs of photons, a linearly polarized Ar + laser, power-stabilized with 30 mW of output at 351,1 nm, was used to pump a KDP crystal (see Figure 1). The input and output ends of the crystal were antireflection-coated for 351 nm and 702 nm, respectively. The crystal was cut so that its optic axis was inclined at 52 to the pump beam direction. The output angles of the correlated photons can be determined from the energy and momentum constraints, the index of refraction data for the crystal and the orientation of the pump beam with respect to the crystal optic axis [1]. This particular configuration yields correlated pairs of photons covering the visible region of the spectrum emitted within a fairly narrow range of angles centred 4 from the pump beam direction. For our four efficiency measurements, we selected pairs of photons at four different pairs of Metrologia, 1995/96, 32, 479-483 479