Parametric cQED: a new approach to realizing strong light-matter interactions
Strong light-matter interactions form the standard paradigm for quantum information processing. Most QIP platforms, however, employ static interactions where both the strength and the form of interactions are `hard-wired’ in the design. In this talk, I will describe a new framework based on time-dependent strong parametric interactions, a.k.a. parametric cQED, which realizes new and powerful functionalities for in-situ tunable qubit readout and control. To analyze such strong parametric couplings, we developed a perturbative Hamiltonian diagonalization technique based on sequential time-dependent Schrieffer-Wolff transformations , which allows accounting for inertial terms at successive orders independently. When applying our technique to canonical cQED setups, we predict astonishingly large dispersive shifts compared to those attainable with static interactions. Crucially, these dressed shifts are tunable in magnitude and sign with the frequency and the amplitude of the pump mediating the interactions; this tunability supports exact cancellation of static and qubit-induced dispersive shifts on the cavity for specific pump frequencies (“blind spots”) even for strong interaction strengths. We find the presence of blind spots to be a robust feature in parametric cQED, under both two-level and multi-level approximations, that can be exploited to realize a readout switch with high on/off ratio. Furthermore, our results indicate that Bloch-Siegert shifts can be rendered large even in the dispersive regime, and hence the validity of the rotating-wave needs to be revisited in the presence of time-dependent interactions. Finally, we identify a new “parametric straddling regime” in multi-level atoms, such as transmons, realizable even when the detuning between the transmon and cavity frequencies is large. In support of our theory, I will present experimental results  showing high-contrast single- and joint-qubit readout in a system comprising two transmons parametrically coupled to a lumped-element cavity.