Coherent spin-qubit photon coupling: Part 2*

Coherent coupling of long-distance spins is a crucial step towards quantum information processing with spin-qubits. We use a circuit quantum electrodynamics architecture to demonstrate strong coupling between single microwave photons in a high impedance NbTiN cavity and a three-electron spin-qubit in a GaAs triple quantum dot. We resolve the vacuum Rabi mode splitting with a coupling strength of g/2π≈31MHz and a qubit decoherence rate of γ₂/2π≈20MHz. The spin-qubit is formed by exchange interaction, which couples the spin with the orbital degrees of freedom. We can directly access the amount of spin-charge coupling strength electrostatically. This allows us to tune both the qubit-photon coupling strength as well as the qubit decoherence. From dispersive two-tone spectroscopy measurements we extract a minimum qubit decoherence rate of γ₂/2π≈10MHz for a coupling strength of g/2π≈23MHz. We also observe an ac Stark shift of the qubit frequency, which allows us to calibrate the number of photons in the resonator and gives direct access to the qubit-photon coupling strength.