Universal control of a 4 singlet-triplet-qubit quantum processor in a germanium quantum dot ladder*

Rich underlying physics and a relatively high-yield fabrication process have enabled Ge/SiGe quantum dot arrays to emerge as a platform for high-fidelity single-spin qubits [1] and singlet-triplet (ST) qubits with all-electrical control of a quantum processor [2]. However, despite the appealing potential, several challenges remain in its implementation, such as readout, control, and scaling of systems with many qubits [3].

In this work, we construct four singlet-triplet qubits in a 2x4 Ge/SiGe quantum dot array. This represents the largest array of ST semiconductor qubits to date, featuring tunable couplings between all sites. We demonstrate coherent control of all qubits i.e. x-axis rotations driven by spin-orbit coupling and z-axis control driven mainly by exchange coupling. The readout is performed via Pauli-spin blockade. We also implement a two-qubit swap gate between the qubit pairs. This is achieved by increasing the exchange coupling between neighbouring ST qubits. Furthermore, we demonstrate the quantum state transfer across the array using a sequence of swaps.

This concludes the characterization of the system and allows us, for example, to use this quantum processor to simulate quantum phase transitions, resonance valence bond states and magnetism of ladder systems.

[1] Hendrickx N. W., et al. Nature, 591 (7851), 580–585, (2021).

[2] Jirovec, D., et al. Nature Materials, 20(8), 1106–1112, (2021).

[3] Vandersypen, L., et al. npj Quantum Inf.3, 34 (2017).