A scalable platform for germanium spin qubits

Spin qubits in quantum dots are promising candidates for building practical quantum computers, as their small size allows to interconnect and integrate many onto a chip. Moreover, the prospect of using state-of-the-art semiconductor manufacturing processes makes spin qubits in quantum dots a very attractive platform for the development of quantum information processing [1, 2]. However, scaling up the number of spin qubits, while also maintaining a high degree of connectivity, requires the fabrication of extensive two-dimensional qubit arrays with a high device yield. In this context, strained germanium quantum wells are an excellent material platform, as it has relaxed lithographical constraints, comes with a built-in, efficient control mechanism, and is a fully foundry-compatible material [1, 2, 3]. Recently, germanium quantum processors scaled to 10 quantum dots in a 3-4-3 configuration [3]. Furthermore, spin qubits defined in germanium quantum wells have demonstrated fast two-qubit logic, as well as high-fidelity single- and two-qubit control [2, 3]. Additionally, they do not require on-chip magnets, and single-qubit gates can be performed using baseband control signals [3].

Here, we demonstrate the recent developments on scaling the number of semiconductor spin qubits in strained germanium quantum wells. We show the operation of an extensible qubit unit cell with up to six qubits and a charge sensor. In particular, we show advances in the characterisation, operation, and performance of our multi-qubit processors, focussing on reproducibility of quantum dot and qubit metrics, including threshold voltages and charge noise. This allows us to reliably increase the quantum processor size, while maintaining high robustness and yield.

[1] Scappucci, G., et al., Nature Reviews Materials 6.10 (2021): 926-943

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

[3] Wang, C-A et al., Science 385, 6707, (2024)