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

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Abstract
We have measured the autocorrelation of the fluorescence of single aromatic molecules in polyethylene at T≊2 K. Assuming intensity fluctuations to be caused by frequency jumps when two-level systems (TLS’s) change states, we show that individual TLS’s can be probed, allowing direct measurement of some of their properties, with minimal recourse to theory and no assumptions about the distribution of TLS parameters. Besides providing a simple confirmation of the tunneling theory of TLS’s, this new method, by eliminating statistical uncertainties, proves that while some TLS’s may be stable for several hours others are modified by transitions of their neighbors. Tunneling times of individual TLS’s are measured directly, as are their temperature dependences, revealing three types of TLS-phonon coupling, of which one is new.