Realizing large, tunable dispersive shifts with parametric couplings – Part II

Engineered dispersive shifts in cavity-QED systems are important for enabling high fidelity qubit measurement, fast logic gates, state preparation and error correction protocols. Standard superconducting circuit-QED systems typically rely on static coupling between qubits and cavities. Here, tunable dispersive shifts are only possible by in-situ frequency tuning of either the qubit or cavity. Manipulation of dispersive shifts in this way can be cumbersome and restricted. With the theoretical backbone for understanding strong parametric dispersive interactions covered in the previous talk, we will focus on describing our experiments with a transmon qubit coupled to a cavity via a dc-SQUID. We show that our system can avoid qubit decoherence by minimizing coupling to the readout cavity during qubit operations. With both the qubit and cavity at fixed frequencies, we can dynamically produce large positive or negative dispersive shifts, achieving high fidelity qubit measurements. In addition to realizing many features of standard cavity-QED, this system also exhibits a unique tunability along with qualitatively new features not supported by static circuit-QED setups, thus opening up a new paradigm for controlling light-matter interactions.