Radio frequency pulsed-gate charge spectroscopy on coupled quantum dots

Time-resolved electron dynamics in coupled quantum dots is directly observed by a pulsed-gate technique. While individual gate voltages are modulated with periodic pulse trains, average charge occupations are measured with a nearby quantum point contact as detector. A key component of our setup is a sample holder optimized for broadband radio frequency applications. Our setup can detect displacements of single electrons on time scales well below a nanosecond. Tunneling rates through individual barriers and relaxation times are obtained by using a rate equation model. We demonstrate the full characterization of a tunable double quantum dot using this technique, which could also be used for coherent charge qubit control. been used for the control of spin qubits. 18-20 However, com- pared to our measurements, the pulse durations and repeti- tion periods reported there were much longer, hence, impos- ing smaller challenges to radio frequency rf components. In addition, these earlier and following works relied on a qubit initialization via charge exchange with the leads, which in- volves pulsing between three different charge configurations of the double QD. In contrast, we pulse between only two configurations and work at a constant overall charge. In this case, charge exchange with the leads is no longer required and the tunnel coupling to the leads can be tuned to be very weak. We expect this effective decoupling from the bath of two-dimensional electrons to ultimately lead to longer coher- ence times. Here, we demonstrate a characterization of the double QD parameters relevant for the control of a charge qubit. We use the same pulsed-gate technique which will be employed to actually operate qubits, hoping to increase the overall efficiency of a future quantum computer. Introducing a rate equation model allows us to obtain from our data tun- neling rates of individual barriers and energy relaxation times. Moreover, we are able to distinguish between possible charge relaxation channels, which can be tuned via gate volt- ages. This is especially important for future qubit applica- tions where specific relaxation channels might be tempo- rarily opened for initialization or readout, while long energy relaxation times are required during coherent manipulation.