Realizing superconducting spin qubits

Josephson junctions implemented in semiconducting nanowires proximitized by a superconductor exhibit intricate physics arising from the interplay of electron-electron interactions, superconductivity, spin-orbit coupling, and the Zeeman effect. This thesis explores these phenomena through a series of experiments conducted using circuit quantum electrodynamics techniques.
After establishing the fundamental theoretical concepts and experimental methodologies, we introduce a crucial element for probing our devices with microwaves: magnetic field-compatible resonators. We then describe various experiments conducted over the past years in which superconducting resonators and other circuits are used to explore the physics of nanowire Josephson junctions.
In an initial experiment, we develop a magnetic-field-resilient fluxoniumcircuit that incorporates an InAs semiconducting nanowire at its core. We show that the device’s spectrum is highly dependent on both the electrostatic gate voltage and the magnetic field strength, allowing us to detect signatures of non-conventional phenomena in semiconducting Josephson junctions.
The bulk of this thesis revolves around a second set of experiments, where a quantum dot is electrostatically defined within the nanowire Josephson junction. This time, we use a transmon circuit to investigate singlet-doublet ground state transitions and their dynamics. The two spinful doublet states of the junction define a novel type of qubit with intriguing properties: a superconducting (or Andreev) spin qubit (ASQ). Thus, we then shift our focus to the doublet states and explore their magnetic field dependence with transmon spectroscopy. Subsequently,we turn to directly investigating the spin-flip transition and the coherence properties of the two spin states. We find that the intrinsic coupling between the spin state and the supercurrent through the junction enables
strong coupling between the ASQ and the transmon qubit in which it is embedded.
In a final experiment, we connect two such Andreev spin qubits in parallel and investigate their supercurrent-mediated longitudinal coupling. We find that the qubits are strongly coupled and their coupling strength can be switched on and off by adjusting the magnetic flux. Notably, given that the spins are placed micrometers apart, this mechanism enables interaction between distant spins. Building on these promising characteristics, we end by introducing a proposal that outlines our vision for scaling up ASQs. The proposed architecture, where multiple ASQs are connected in parallel, enables the selective coupling of any pair of qubits in the system, regardless of their spatial separation, through flux control.
This thesis concludes by outlining potential future experiments that could be conducted with devices and techniques similar to those investigated here.