Gate-based readout of hybrid quantum dot systems

Abstract

Quantum mechanics yields exciting opportunities for developing novel technologies. In particular, quantum computation enables performing otherwise intractable calculations. However, unwanted disturbances to the quantum bits (qubits) form a formidable challenge for its implementation. Topologically encoding information protects against these disturbances. Qubits based on Majorana zero modes are promising for achieving topological protection and form a model application for the results of this thesis. This thesis focuses on gate-based sensing, a scalable and high-fidelity readout mechanism for solid-state quantum information processing devices. We investigate semiconductor/superconductor hybrid quantum dot devices implemented in InAs nanowires. Radio frequency (RF) techniques allow rapid and multiplexed measurements of mesoscopic systems without relying on DC-transport. As such, we show that RF measurements provide a vital tool for rapid readout and quick tune-up of semiconductor qubits. We start by presenting the theoretical foundations of quantum dots and resonators, necessary for describing the subsequent experimental results. Next, we provide relevant details concerning the experiments in this thesis. The first experiment shows the implementation of dispersive gate sensing (DGS) in a semiconductor double quantum dot (DQD). We show dispersive shifts on the order of the resonator linewidth and study its behavior for different readout powers. These shifts match theoretical expectations and allow differentiating between Coulomb blockade and resonance with a signal-to-noise ratio (SNR) of 2 within \SI{1}{\micro\second}. We subsequently apply DGS to a semiconducting quantum dot coupled to a superconducting island and observe spin-dependent tunneling and simultaneous two-particle tunneling involving Cooper pairs. By inhibiting electron tunneling to the outside leads, we bring the system to an otherwise inaccessible regime and show that DGS can probe floating systems. The third experiment replaces the MHz-resonators with on-chip superconducting coplanar waveguide resonators in the GHz regime. We extract the differential conductance quantitatively without relying on any DC calibration data. Furthermore, we obtain an SNR of 15 in 1 microsecond distinguishing Coulomb blockade from resonance in a semiconductor DQD. In the final experiment, we combine the preceding experimental results and investigate a superconducting island between two semiconductor quantum dots. We can split single Cooper pairs on demand with this geometry while retaining the resulting electrons. Secondly, we measure the electron parity using gate-based sensing in a DQD without external charge sensors. This thesis concludes by discussing the relevance of the obtained experimental results to the Majorana box qubit and suggestions for subsequent experiments. The results of this thesis show that gate-based sensing is a versatile tool in the context of mesoscopic experiments and quantum information processing devices in particular.BUS/Quantum Delf