Detecting single-electron tunneling involving virtual processes in real   time
  We use time-resolved charge detection techniques to probe virtual tunneling processes in a double quantum dot. The process involves an energetically forbidden state separated by an energy $\delta$ from the Fermi energy in the leads. The non-zero tunneling probability can be interpreted as cotunneling, which occurs as a direct consequence of time-energy uncertainty. For small energy separation the electrons in the quantum dots delocalize and form molecular states. In this regime we establish the experimental equivalence between cotunneling and sequential tunneling into molecular states for electron transport in a double quantum dot. Finally, we investigate inelastic cotunneling processes involving excited states of the quantum dots. Using the time-resolved charge detection techniques, we are able to extract the shot noise of the current in the cotunneling regime. 
S. Gustavsson, M. Studer, R. Leturcq, T. Ihn, K. Ensslin, D. C.   Driscoll, A. C. Gossard