Graphene quantum dots are attractive candidates for solid-state quantum bits. In fact, thepredicted weak spin-orbit and hyperfine interaction promise spin qubits with long coherencetimes. Graphene quantum dots have been extensively investigated with respect to theirexcitation spectrum, spin-filling sequence and electron-hole crossover. However, theirrelaxation dynamics remain largely unexplored. This is mainly due to challenges in devicefabrication, in particular concerning the control of carrier confinement and the tunability ofthe tunnelling barriers, both crucial to experimentally investigate decoherence times. Here wereport pulsed-gate transient current spectroscopy and relaxation time measurements ofexcited states in graphene quantum dots. This is achieved by an advanced device design thatallows to individually tune the tunnelling barriers down to the low megahertz regime,while monitoring their asymmetry. Measuring transient currents through electronic excitedstates, we estimate a lower bound for charge relaxation times on the order of 60–100 ns.
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