Quantum dot occupation and electron dwell time in the cotunneling regime

We present comparative measurements of the charge occupation and conductance of a GaAs/AlGaAs quantum dot (QD). The dot charge is measured with a capacitively coupled quantum point-contact sensor. In the single-level Coulomb blockade regime near equilibrium, charge and conductance signals are found to be proportional to each other. We conclude that in this regime, the two signals give equivalent information about the QD system. Out of equilibrium, we study the inelastic-cotunneling regime. We compare the measured differential dot charge with an estimate assuming a dwell time of transmitted carriers on the dot given by h/E, where E is the blockade energy of first-order tunneling. The measured signal is of similar magnitude as the estimate, compatible with a picture of cotunneling as transmission through a virtual intermediate state with a short lifetime.