Anisotropic exchange interaction induced by a single photon in semiconductor microcavities

We investigate coupling of localized spins in a semiconductor quantum dot embedded in a microcavity. The lowest cavity mode and the quantum dot exciton are coupled and close in energy, forming a polariton. The fermions forming the exciton interact with localized spins via exchange. Exact diagonalization of a Hamiltonian in which photons, spins, and excitons are treated quantum mechanically shows that a single polariton induces a sizable indirect anisotropic exchange interaction between spins. At sufficiently low temperatures strong ferromagnetic correlations show up without an appreciable increase in exciton population. In the case of a (Cd,Mn)Te quantum dot, Mn-Mn ferromagnetic coupling is still significant at 1 K: spin-spin correlation around 3 for exciton occupation smaller than 0.3. We find that the interaction mediated by photon-polaritons is 10 times stronger than the one induced by a classical field for equal Rabi splitting.

G.C. and J.F.R. acknowledge financial support from the Ramón y Cajal Program (MCyT, Spain) and from CAV, Grant No. GV05/152. Partial financial support by the Spanish MCYT (Grant Nos. MAT2002-04429-C03, MAT2003-08109-C02-01, and FIS200402356), the Brazilian agencies FAPERJ, CAPES, and CNPq, and the Argentinian UBACYT and CONICET.