  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.
