Integrated Readout at the Quantum-Classical Interface of Semiconductor Qubits

Abstract

Quantum computing promises to deliver uniquely powerful information processing machines by exploiting the quantum phenomena of superposition and entanglement. In solid-state systems, there has been significant progress in the isolation and control of the fundamental units needed to build such machines, known as qubits. However, scaling-up the number of qubits to the point where sophisticated algorithms can be performed presents considerable experimental challenges. In particular, it is becoming increasingly apparent that a new class of tools will be required to interface between fragile quantum systems, and the classical readout and control hardware of the outside world. This thesis presents experimental investigations towards the development of a scalable readout architecture for semiconductor qubit platforms. Fast readout of a GaAs-AlGaAs double quantum dot in the few-electron regime is first demonstrated via an embedded dispersive gate sensor (DGS), alleviating the burden of requiring separate charge sensors for every qubit. The sensitivity and bandwidth of this technique are extracted and benchmarked against well-established readout methods. Dispersive gate sensing of quantum point contacts (QPCs) is then presented, probing charge rearrangement within the local electrostatic environment of quasi one-dimensional channels. A low-loss, lumped-element, LC resonant circuit is also implemented for frequency multiplexed readout. The second set of experiments concern the design and characterisation of miniaturised, on-chip circulators based on the quantum Hall effect, and the quantum anomalous Hall effect. Microwaves are first capacitively coupled into edge magnetoplasmon modes in a mesoscopic GaAs-AlGaAs droplet. Non-reciprocal forward transmission comparable to off-the-shelf components is observed, which is accounted for within an interferometric picture. This circulator design is then extended to thin films of the three-dimensional topological insulator, Cr-doped (Bi,Sb)2Te3, wherein similar non-reciprocity is demonstrated in the absence of an external magnetic field.