Strong coupling, energy splitting, and level crossings: A classical perspective

摘要

The interaction between quantum systems is commonlynstudied in the weak coupling and the strong coupling limits.nWeak coupling can be treated within perturbation theory, andnvarious classical analogs have been developed.1–3 Examplesnof the effects of weak coupling are changes in atomic decaynrates4 u0001Purcell effectu0002 and Förster energy transfer5 between andonor and acceptor atom or molecule. Förster energy transfernassumes that the transfer rate from donor to acceptor isnsmaller than the relaxation rate of the acceptor. This assumptionnensures that once the energy is transferred to the acceptor,nthere is little chance of a back transfer to the donor. Oncenthe interaction energy becomes sufficiently large, and a backntransfer to the donor becomes possible, the system is in thenstrong coupling regime. In this limit, it is no longer possiblento distinguish between donor and acceptor. Instead, the excitationnbecomes delocalized, and we must view the pair asnone system. A characteristic feature of the strong couplingnregime is energy level splitting, a property that can be wellnunderstood from a classical perspective.