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 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.
Within four years time, this platform  has rapidly evolved from materials growth to supporting multi-qubit logic . I will discuss the development of this system, starting from material growth and characterization  to recent results on operating a highly-connected two-dimensional qubit array . We implement qubit logic all electrically and the exchange interaction can be pulses independently to freely program one, two, three, or four qubit gates. Furthermore, we show that we can extend the quantum coherence by several orders of magnitude by implementing dynamical decoupling. All these techniques are combined to perform a quantum circuit that generates a four-qubit Greenberger-Horne-Zeilinger state, showcasing coherent operation of all four qubits together. This positions strained germanium as a unique material for quantum applications. I will furthermore discuss strategies, challenges, and opportunities in scaling these systems up as a step towards the realization of scalable qubit tiles for fault-tolerant quantum processors.
 Scappucci, G. et al. The germanium quantum information route. Nature Reviews Materials (2020);
 Hendrickx N.W. et al. Fast two-qubit logic with holes in germanium. Nature 577 487-491 (2020);
 Sammak, A. et al. Shallow and Undoped Germanium Quantum Wells: A Playground for Spin and Hybrid Quantum Technology. Advanced Functional Materials 29 14 (2019);
 Hendrickx N.W. et al. A four-qubit germanium quantum processor. Nature 591 580-585 (2021).
Nico Hendrickx graduated in experimental physics from the University of Twente. He received his PhD cum laude for his work on hole spin qubits in strained germanium quantum wells at the Delft University of Technology.
Next, he moved to IBM Research Zurich, to continue working on spin qubit systems. His current research interests include coherence of hole spin states and scaling quantum dot systems, working towards large-scale quantum computation with semiconductors.