Konwersatorium im. Leopolda Infelda

By confining photons in a semiconductor microcavity, and strongly coupling them to electronic excitations, one may create polaritons: bosonic quasi-particles with an effective mass of 10-9 times that of Rubidium atoms, thus allowing Bose-Einstein condensation at elevated temperatures. After a long and strenuous search, the comprehensive set of experiments has finally given evidence for BEC of polaritons. However, this new condensate depart from the archetypal BEC in several ways. Most importantly, polaritons have short lifetime and so a continues pumping is required to sustain a steady-state. This leads to a new type of condensation which can exist in highly non-equilibrium and dissipative environment. Collective behaviour of these exotic particles brings together a wide range of concepts from condensed matter and optical physics: the crossover between BEC of already formed bosons and BCS pairing of fermions, phase transitions in non-equilibrium systems, decoherence, structural disorder and low dimensionality. I will discuss these theoretical concepts and review the experimental state of the art in the field.