Realizing an Andreev Spin Qubit

The thesis gives the first experimental demonstration of a new quantum bit ("qubit") that fuses two promising physical implementations for the storage and manipulation of quantum information - the electromagnetic modes of superconducting circuits, and the spins of electrons trapped in semiconductor quantum dots - and has the potential to inherit beneficial aspects of both. This new qubit consists of the spin of an individual superconducting quasiparticle trapped in a Josephson junction made from a semiconductor nanowire. Due to spin-orbit coupling in the nanowire, the supercurrent flowing through the nanowire depends on the quasiparticle spin state. This thesis shows how to harness this spin-dependent supercurrent to achieve both spin detection and coherent spin manipulation. This thesis also represents a significant advancement to our understanding and control of Andreev levels and thus of superconductivity. Andreev levels, microscopic fermionic modes that exist in all Josephson junctions, are the microscopic origin of the famous Josephson effect, and are also the parent states of Majorana modes in the nanowire junctions investigated in this thesis. The results in this thesis are therefore crucial for the development of Majorana-based topological information processing.

Auteur

Max Hays was born in Asheville, North Carolina in the United States. He received his BS in Physics from the University of North Carolina at Chapel Hill, where he worked on a neutrinoless double-beta decay experiment known as the MAJORANA demonstrator. In the fall of 2014, Max joined the lab of Michel Devoret at Yale University, where he performed the experiments presented in this thesis. He currently lives in Cambridge, Massachusetts with his wife Radha, and works in the Engineering Quantum Systems group led by Will Oliver at MIT.

Contenu

Part 1: Key concepts and contributions.- Chapter 1: Introduction.- Chapter 2: Andreev levels.- Chapter 3: Probing Andreev levels with cQED.- Chapter 4: Unlocking the spin of a quasiparticle.- Chapter 5: Future directions.- Part 2 The beautiful, messy details.- Chapter 6: BCS superconductivity.- Chapter 7: Andreev reflection, Andreev levels, and the Josephson effect.- Chapter 8: Andreev levels in Josephson nanowires.- Chapter 9: What would happen in a topological weak link?.- Chapter 10: The device.- Chapter 11: Spectroscopy and dispersive shifts.- Chapter 12: Raman transitions of the quasiparticle spin.- Chapter 13: Interactions of Andreev levels with the environment.- Chapter 14: Unexplained observations.