In-situ Tuning of the Electric-Dipole Strength of a Double Dot Charge Qubit: Charge Noise Protection and Ultra Strong Coupling*

Semiconductor quantum dots (QDs) are promising building blocks for semiconductor-based quantum technology. Here, we investigate double-QD (DQD) charge qubits in GaAs, capacitively coupled to high-impedance SQUID array and Josephson junction array resonators. We tune the strength of the electric-dipole interaction between the qubit and the resonator in-situ using surface gates. We characterize the qubit-resonator coupling strength, qubit decoherence, and detuning noise affecting the charge qubit for different electrostatic DQD configurations. We find that all quantities can be tuned systematically over more than one order of magnitude, resulting in reproducible decoherence rates < 5 MHz in the limit of high inter-dot capacitance. Conversely, by reducing the inter-dot capacitance, we can increase the DQD electric-dipole strength, and therefore its coupling to the resonator. By employing a Josephson junction array resonator with an impedance of 4 kOhm and a resonance frequency of 5.6 GHz, we observe a coupling strength of 630 MHz, demonstrating the possibility to achieve the ultrastrong coupling regime (USC) for electrons hosted in a semiconductor DQD. These results are essential for further increasing the coherence of QD-based qubits and investigating USC physics in semiconducting QDs.