Microscopy and Spectroscopy of Topological Quantum Devices

Aims of the project: Probing how electrons behave in nanoelectronic devices has revolutionised our understanding of quantum systems and laid the foundations for a wide range of solid-state quantum technologies. This experimental PhD project involves probing a new class of quantum device whose electronic properties are governed by bandstructure topology. Electrons in these systems are forced to move along surfaces and edges where they avoid bulk noise and disorder, potentially improving the performance of quantum devices like qubits and metrological standards. The main aims of the project are:

1. Detecting Majorana zero modes in topological superconductors: Academic teams and technology companies such as Microsoft are developing a topological quantum computer using a special type of delocalised electron - a Majorana zero mode (MZM) - that forms at the ends of superconductor-semiconductor nanowires. MZMs have so far only been observed indirectly and never spatially resolved microscopically. Scanning gate microscopy, which uses the tip of an atomic force microscopic as a mobile local gate electrode, will be used to image their position as a function of magnetic field and gate voltage. Circuit quantum electrodynamics (cQED) will be used to spectroscopically detect MZMs embedded in superconducting qubits. The impact of this study is potentially very high as it will verify the spatial location of MZMs, one of their defining features.
2. Imaging topological gaps in magnetically-doped topological insulator devices: We will explore magnetically-doped V-VI compounds and take images of the spatial distribution of currents as two-dimensional surface states evolve into the one-dimensional edge channels. Capturing images at different carrier density and magnetic field should reveal precisely how this transition takes place, and by comparing with topography we can understand how it is affected by ionised surface and bulk impurities, edge disorder, and variations in crystal morphology (see https://magma.tmqs.lu/ for more details).
Techniques, activities, and equipment used: You will use a range of low temperature scanning probe microscopy techniques to image electrons at the nanoscale and verify these topological properties. You will also learn techniques such as circuit quantum electrodynamics, atomic force microscopy, scanning electron microscopy, electron-beam lithography and low-temperature transport measurements. Millikelvin in-operando scanning gate microscopy, electrical transport measurements, and microwave spectroscopy will be performed using the Quantum Science and Device Facility at Imperial.