Individual localized emitters embedded in a solid state matrix, like excitons conﬁned in quantum dots, represent a model system for testing solid state quantum information processing schemes. A prerequisite of such activity is an accurate measurement, manipulation and modeling of coherent, nonlinear responses of single excitons.
The term “coherence” usually relates to the presence of a spatio-temporal phase relation within the physical ﬁeld, which commonly is manifested by the appearance of an interference pattern. Conversely, quantum emitters are localized on sub-wavelength areas, and, as such, are not capable of producing spatially extended interferences. In the ﬁrst part of this lecture I will present an intriguing and unique concept of retrieving coherence properties of individual emitters. This breakthrough was achieved with heterodyne spectral interferometry technique  allowing the retrieval of resonant nonlinear responses, like four-wave mixing (FWM).
In the second part I will highlight our recent advances in the FWM spectroscopy of a single, strongly-conﬁned exciton-biexciton system . I will discuss homogenous and inhomogeneous broadening and dephasing mechanisms of an exciton transition. I will report on observation of phonon-induced dephasing and polaron formation in a single quantum dot. I will present text-book examples of coherent dynamics, time-resolved
polarization state and polarization selection rules.
For the remaining part I will consider an individual exciton strongly-coupled with a photon mode of a pillar microcavity. By performing FWM spectroscopy I will demonstrate the Jaynes-Cummings nonlinearlity in this system . Speciﬁcally, I will show that the polariton splitting increases as a square-root of number of photons present in the cavity.
As an outlook, I will discuss the prospects of quantum bus technology employing semiconductor nanostructures.
 W. Langbein and B. Patton Opt. Lett. 31, 1738 (2006)
 J. Kasprzak et al. to be published
 J. Kasprzak et al. Nature Mater. 9, 304 (2010).