Spin-photon coupling using circular double quantum dots

Abstract: We propose and analyze a microwave spin-photon interface based on a circular double quantum dot, inspired by recent experimental observations of anisotropic g-factors and ring states in InAs nanowires. We develop an effective theoretical model capturing the interplay of spin-orbit coupling and the magnetic flux through the ring and show how ring states form at crossings of odd and even geometrical parity orbital states. Similar to bonding and antibonding states of conventional double quantum dots, the ring eigenstates can be changed into single dot states by detuning the dots, which enables a high degree of control over the system's properties. Applying a tilted magnetic field induces spin-charge hybridization which enables spin-photon coupling. For low disorder, the photons couple states of simultaneously (almost) opposite spin and angular momentum. With increasing disorder, the spin-photon coupling becomes analogous to the flopping mode mechanism of conventional double quantum dots where the spin is hybridized with the bonding and antibonding orbital states without angular momentum. We show that the system exhibits a second-order charge-noise sweet spot at a specific magnetic field angle, which lowers the system's sensitivity to dephasing while retaining a substantial spin-photon coupling strength. Moreover, the photon coupling mechanism can be switched off either electrically, by detuning to the single-dot regime, or magnetically, by rotating the field to disable the spin-charge hybridization.