Martin van Exter is one the staff members of the Quantum Optics group in Leiden. His research topics include (i) quantum entanglement of photons, (ii) quantum tomography and superconducting detectors, (iii) cavity quantum electrodynamics, (iv) surface plasmon physics and (v) spatial correlations in optics, and its relation to scattering and absorption. The 'Quantum Optics' group in Leiden has been active in this field for more than 20 years. It started in the mid eigthies under the leadership of prof. Han Woerdman, with Eric Eliel and Jo Hermans in experimental optics and Gerard Nienhuis as theoretical professor and occasional support from Daan Lenstra and Gert 't Hooft. Martin van Exter joined in 1990 and Michiel de Dood and Dirk Bouwmeester joined at the beginning of this century. Martin, Michiel and Dirk currently form the core of the Quantum Optics group in Leiden.