Optimizing coherence of germanium spin qubits.*

The prospect of building quantum circuits using advanced semiconductor manufacturing techniques position quantum dots as an attractive platform for quantum information processing. Initial demonstrations of one and two-qubit logic have been performed in gallium arsenide and later silicon. However, until recently, interconnecting larger spin qubit systems has remained a challenge.

Over the past years, hole states in strained germanium quantum wells have emerged as a viable host for spin qubits. These states have favourable properties for defining extended spin qubit arrays. The small effective mass relaxes constraints on lithography, the low degree of disorder enables reproducible quantum dots, the lack of a valley degeneracy ensures an well-defined qubit state and the strong spin-orbit coupling allows for local and electrical qubit control.

This platform has rapidly evolved from materials growth to supporting multi-qubit logic. I will give an overviewof recent developments in this system, starting from material growth to recent results on operating a highly-connected two-dimensional qubit array. Next, I will discuss the impact of noise on the hole qubit coherence, as well as strategies to mitigate this. We study the magnetic field dependence of various qubit properties in order to find sweet spots for operation. Finally, I will discuss strategies, challenges, and opportunities in scaling these systems up as a step towards the realisation of scalable qubit tiles for fault-tolerant quantum processors.