High impedance circuit quantum electrodynamics with hole spins

There has been impressive progress in the past 10 years in the field of spin qubits for what concerns their key properties, namely the spin relaxation and dephasing times. The necessity for scaling up, i.e. coupling many qubits to each other, becomes thus more and more important. One of the most promising approaches is to couple two spins with a superconducting resonator, via the so-called spin- photon coupling. In this project we aim to study hole spins localized in Ge nanostructures and use superconducting high impedance resonators in order to reach the strong coupling regime for the first time between a hole spin and a single photon. More concretely, we will fabricate Ge quantum devices hosting hole spins and couple them to superconducting resonators fabricated out of granular Aluminum which has shown very promising properties as a high impedance resonator. Initially the resonators are going to be used to readout the qubit states before moving towards the strong -spin photon coupling. During the duration of the project there will be continuous exchange between the experimentalists and the theorist not just for interpreting the results but also to predict novel effects. By combining the know-how of superconducting circuits, semiconductor quantum dot devices and condensed matter theory we aim to successfully tackle all the challenges.

Holes confined in Ge/SiGe heterostructures have emerged as a very promising platform for realizing spin qubits. One of the key properties that makes holes interesting is the spin-orbit interaction, as it allows the driving of spins with oscillating electric fields. In this project, we have been fabricating double quantum dot devices in such Ge/SiGe heterostructures. By studying singlet-triplet qubits, we investigated the effect of the cubic Rashba spin-orbit interaction on the mixing of spin states. In addition, we used these double quantum dot devices to study charge-photon coupling. By developing an ohmmeter capable of operating under high vacuum conditions, we were able to create high-impedance resonators by evaporating aluminum in the presence of oxygen. High-impedance resonators exceeding the resistance quantum could be realized. By integrating an 8 K granular aluminum resonator with such a double quantum dot device in planar Ge, we demonstrated a record hole charge-photon coupling strength. Due to the magnetic field resilience of the developed resonators, these results open the path toward strong spin-photon coupling and long-distance two-qubit gate experiments.