Amplified and tunable transverse and longitudinal spin-photon coupling in hybrid circuit-QED

We describe a method to tune, in situ, between transverse and longitudinal light-matter coupling in a hybrid circuit-QED device composed of an electron-spin degree of freedom coupled to a microwave transmission line cavity. Our approach relies on periodic modulation of the coupling itself, such that in a certain frame the interaction is both amplified and either transverse or, by modulating at two frequencies, longitudinal. The former realizes an effective simulation of certain aspects of the ultra-strong-coupling regime, while the latter allows one to implement a longitudinal readout scheme even when the intrinsic Hamiltonian is transverse, and the individual spin or cavity frequencies cannot be changed. We analyze the fidelity of using such a scheme to measure the state of the electron-spin degree of freedom, and argue that the longitudinal readout scheme can operate in regimes where the traditional dispersive approach fails.