  Graphene quantum dots are attractive candidates for solid-state quantum bits.
In fact, the predicted weak spin-orbit and hyperfine interaction promise spin
qubits with long coherence times. Graphene quantum dot devices have been
extensively investigated with respect to their excitation spectrum,
spin-filling sequence, and electron-hole crossover. However their relaxation
dynamics remain largely unexplored. This is mainly due to challenges in device
fabrication, in particular regarding the control of carrier confinement and the
tunability of the tunnelling barriers, both crucial to experimentally
investigate decoherence times. Here, we report on pulsed-gate transient
spectroscopy and relaxation time measurements of excited states in graphene
quantum dots. This is achieved by an advanced device design, allowing to tune
the tunnelling barriers individually down to the low MHz regime and to monitor
their asymmetry with integrated charge sensors. Measuring the transient
currents through electronic excited states, we estimate lower limit of charge
relaxation times on the order of 60-100 ns.
