Strong coupling of a microwave photon to spin and charge qubits in GaAs quantum dots

We demonstrate the strong coupling limit of cavity QED with individual charges and spins in GaAs quantum dots by using the enhancement of the electric component of the vacuum fluctuations of a resonator with impedance beyond the typical 50 Ohm of standard coplanar waveguide technology. In a first experiment [1], we have realized a frequency - tunable microwave resonator with high impedance implemented by using the large inductance of a SQUID array combined with a small stray capacitance. Its inductance, and thus also its impedance and resonance frequency, is tunable by applying a small magnetic field using a mm-sized coil mounted on the sample holder. In the resonant regime, we resolve the vacuum Rabi mode splitting of 238 MHz at a resonator linewidth 12 MHz and a charge qubit decoherence rate of 40 MHz extracted independently from microwave spectroscopy in the dispersive regime. In a second experiment [2], we couple a spin qubit in a GaAs triple quantum dot to a high impedance NbTiN superconducting resonator which can sustain finite magnetic fields. We resolve the vacuum Rabi mode splitting with a coupling strength of 31 MHz and a spin-qubit decoherence rate of 20 MHz. We can tune coupling and decoherence electrostatically and obtain a minimal decoherence rate of 8.6 MHz. We directly measure the dependence of coupling strength on the tunable electric dipole moment of the qubit using the ac Stark effect.
[1] A. Stockklauser, P. Scarlino, et al., Phys. Rev. X 7, 011030 (2017)
[2] A. Landig, J. Koski, P. Scarlino et al., arXiv