Quantum coherence benchmarks and coherent control of hole spin qubits in a 2x2 germanium quantum dot array

The spin state of an elementary charge is a well-established candidate for quantum information processing. Multiple semiconductor platforms are presently under considerable study to determine their viability as host materials for spin qubits, in particular their suitability as hosts of scalable qubit unit cells.
Germanium quantum wells in planar heterostrucutres (Ge/SiGe) have recently emerged as a highly promising platform in which to host heavy-hole based spin qubits. Demonstrations of the first quantum dots, two qubit logic, fast single qubit operations with fault tolerant fidelities, and even the operation of two dimensional arrays have occurred within the relatively short time span of 3 years.
Here, we build on this momentum, reporting hole spin qubits with the longest spin relaxation times and dephasing times ever measured in the platform, and investigate their magnetic field dependence, concluding that spin decoherence does not constitute a road block for quantum information processing with hole spins in germanium. We also present measurements in coherent control of hole spin qubits in a 2x2 quantum dot array, constituting a milestone in semiconductor based quantum computing.