We have investigated the microwave frequency dynamic admittance of a quantum dot tunnel coupled to a two-dimensional electron gas. The measurements are made via a high-quality 6.75 GHz on-chip resonator capacitively coupled to the dot. The resonator frequency is found to shift both down and up close to conductance resonances of the dot corresponding to a change in the reactance of the system from capacitive to inductive (see Fig. 1). The observations are consistently explained in a scattering matrix model.

Tomography is the main method used for measuring the fidelity of an experimentally implemented quantum process. However, it is a very inefficient method since the number of measurements as well as the time needed for the data post-processing scale exponentially with the number of qubits. With the ongoing experimental progress and growth in system size, quantum process tomography will soon become infeasible in state-of-the art experiments.

Researchers from the Kavli Institute of Nanoscience (Delft University of Technology) and the FOM Foundation have devised a new method for reading out the electron state in a carbon nanotube. They did this by 'stamping’ an individual nanotube on a chip. Then the electron state can be read out, as the nanotube remains very clean and controllable. Electrons in such a nanotube can acquire special quantum characteristics which make these structures interesting for future quantum computers. The readout of the electron states is a crucial step in this.