Modulation of Quality Factors of a Coplanar Superconducting Resonator for Semiconductor Qubit Read-out

For gated semiconductor quantum dot qubits, coplanar superconducting resonators are now being used both for the coupling of distant qubits and for fast dispersive read-out. For the dispersive read-out, it is desirable to control the quality factors of the resonator to properly set the detection bandwidth and the internal microwave-photon dissipation. We have used a small patch of a two-dimensional electron gas, in the proximity of a quarter-wavelength superconducting resonator, to modulate the quality factors of the resonator. The experiment was done on a Si/SiO2 hetero-structure device with patterned aluminum gates which is operated in depletion mode. By measuring both in-phase and quadrature parts of the reflection signal, we found both the internal and external quality factors can be substantially changed when a gate voltage on the electron gas is applied. In addition, the resonant frequency can be altered as well. We will present a model to understand how the electron gas can modulate the Q-factors and effectively control bandwidth and dissipation of the read-out resonator.