A method of calibrating an optical detector arrangement (38,42) comprises simultaneously generating a plurality of entangled photon pairs, such that one photon from each pair traverses a first path (36-38-42) and the other photon from each pair traverses a second path (36-40-44). The number of photons received along the first path is calculated using the detector arrangement (38,42), while the number of simultaneously-generated photons received along the second path is calculated using a second detector arrangement (40,44). These photon numbers are used to calculated an estimate of the detection efficiency (50) of the first detector arrangement (38,42).