Strong charge-photon coupling in Germanium enabled by granular aluminium superinductors

State-of-the-art quantum computers, with roughly a thousand qubits, face a crucial technological challenge of scaling up. Spins confined in quantum dots (QDs) are a promising candidate for qubits due to their long coherence, tunability, control, and readout. However, their natural coupling is the short-ranged (∼ 100 nm) exchange interaction, limited to nearest neighbours. Long-ranged (∼ 1 mm) qubit interactions mediated by a photon could be engineered through a coherent spin-photon coupling. Achieving a strong coupling to a photon is inherently challenging in QDs due to the small dipole moment of the confined charge. However, the potential of high-impedance resonators to compensate for this has gained significant attention in the past decade. Nevertheless, previous QD circuit quantum electrodynamics implementations have not exceeded the impedance of ∼ 3.8 kΩ, leaving opportunities for significant improvement. The large kinetic inductance of granular aluminium (grAl) could provide an order-of-magnitude enhancement. However, fully exploiting the potential of disordered or granular superconductors is challenging as their impedances close to the superconductor-to-insulator transition are difficult to control reproducibly. We report on the realization of a wireless ohmmeter which allows in situ resistance measurements during film deposition and, therefore, indirect control of the kinetic inductance of grAl films. This allows us to reproducibly fabricate resonators with characteristic impedance exceeding the resistance quantum, even reaching 22.3 kW, due to the large sheet kinetic inductance of up to 3 nH □−1 . By integrating an 8 kW resonator with a germanium double QD, we demonstrate a strong charge-photon coupling with the highest rate reported, 566 MHz. The demonstrated method and grAl properties make these resonators suitable for boosting the spin-photon coupling strength, a crucial requirement for fast, high-fidelity, long-distance two-qubit gates.