In this seminar I shall discuss several experimental and theoretical investigations of coupling strength between propagating light beams and single emitters. We show that a single dye molecule can strongly attenuate a laser beam and discuss the conditions for reaching this fascinating regime [1-3]. We find that such an efficient light-matter coupling allows several nonlinear processes to be achieved at low photon levels and present experimental results on the observation of single-molecule Rabi oscillations [4] and stimulated emission from a single molecule which can act as an optical transistor [5]. In the last part of the talk, I shall present our recent progress on the coupling of two molecules at large distances, the preparation of indistinguishable photons from independent sources [6], and the demonstration of the Hong-Ou-Mandel two-photon interference of single photons [7].

[1] G. Zumofen, N.M. Mojarad, V. Sandoghdar, and M. Agio, Phys. Rev. Lett. 101, 180404 (2008).
[2] G. Wrigge, I. Gerhardt, J. Hwang, G. Zumofen, and V. Sandoghdar, Nature Phys. 4, 60-66 (2008).
[3] I. Gerhardt, G. Wrigge, P. Bushev, G. Zumofen, M. Agio, R. Pfab, and V. Sandoghdar,
Phys. Rev. Lett. 98, 033601 (2007).
[4] I. Gerhardt, G. Wrigge, G. Zumofen, J. Hwang, A. Renn, V. Sandoghdar, Phys. Rev. A 79, 011402(R) (2009).
[5] J. Hwang, M. Pototschnig, G. Zumofen, R. Lettow, S. Götzinger, V. Sandoghdar, Nature 460, 76 (2008).
[6] R. Lettow, V. Ahtee, R. Pfab, A. Renn, E. Ikonen, S. Götzinger, V. Sandoghdar
Opt. Express 15, 15842 (2007).
[7] R. Lettow, Y. Rezus, G. Zumofen, A. Renn, E. Ikonen, S. Götzinger, V. Sandoghdar, in preparation.