Probing individual two-level systems in a polymer by correlation of single molecule fluorescence

The autocorrelation of the fluorescence of single aromatic molecules in polyethylene at T approx 2K was measured. Intensity fluctuations are caused by frequency jumps when two-level systems (TLSs) change states. Individual TLSs can be probed with minimal recourse to theory and no assumptions about the distribution of TLS parameters. Besides providing a simple confirmation of the tunneling theory of TLSs, this new method, by eliminating statistical uncertainties, proves that while some TLSs may be stable for several hours others are modified by transitions of their neighbors. Tunneling times of individual TLSs are measured directly. Their temperature dependences reveal three types of TLS-phonon coupling, of which one is new.