Spinful Andreev States in Superconducting Circuits

Abstract

The growing understanding of the physics of superconductor-semiconductor nanostruc- tures is a key driver for the development of emerging quantum technologies. The elemen- tary excitations of these hybrid nanostructures are Andreev bound states. To further our knowledge about their intricate physics, new tools must be used to study them. This thesis describes the use of magnetic-field compatible superconducting circuits to study and ma- nipulate Andreev bound states and their spin in hybrid superconducting-semiconducting nanowire Josephson junctions.
First, we provide an introduction to the physical models describing Andreev bound states in superconducting circuits and the general methodology used for the circuit design, device fabrication and experimental setups in the experiments of this work.
We then move on to an initial set of two experiments in Chapters 4 and 5, where we inductively shunt a superconducting resonator with a nanowire-based radio-frequency su- perconducting quantum interference device (rf-SQUID). This allows us to study Andreev bound states in InAs/Al nanowire Josephson junctions using circuit quantum electrody- namics techniques under various external conditions.
In Chapter 4 we use pulsed detection of Andreev bound state parity to demonstrate par- ity selective spectroscopy. The main result of this Chapter was the discovery of microwave- induced parity polarization, that allows one to set the bound state parity in-situ using microwave pulses. We then study the evolution of the microwave spectrum of Andreev bound states in a magnetic field in Chapter 5. Here we find a multitude of phenomena that arise because of the rich interplay between spin-orbit coupling, the Zeeman effect, super- conductivity, and electron-electron interactions. We observe evidence of spin-polarizing microwave transitions, the anomalous Josephson effect, and transitions involving triplet Andreev spins.
In Chapter 6 we explore an alternative material and junction fabrication method in combination with the same circuitry. Specifically, we excite Andreev bound states in InS- b/Al Josephson junctions defined by shadow-wall lithography. We observe low density, high-transparency Andreev bound states in a range of devices and reproduce the directly spin-polarizing microwave transition observed in Chapter 5. The results of this Chap- ter demonstrate the viability of combining hybrid circuit quantum electrodynamics with advanced material combinations and fabrication geometries.
In the final experiment, Chapter 7, we move back to InAs/Al based junctions. This Chapter uses previous results from Chapter 5 and works demonstrating the use of a single superconducting spin as a quantum bit, as a stepping stone. Here, we embed two super- conducting spin qubits in a single SQUID and demonstrate strong longitudinal coupling between them over a distance much larger than their wavelengths.
The results and methods developed in this dissertation pave the way for continued ex- ploration of the intricating physics of superconducting spins and demonstrate early steps towards their use as a new platform for quantum computing.