David McKay, University of Chicago
Superconducting Josephson‐junction (JJ) qubits are an emerging technology for quantum information processing. These qubits can be engineered with strong coupling to two or three‐dimensional microwave cavities which implements the cavity quantum electrodynamics (QED) paradigm ‐ coherent coupling of a two‐level system to a harmonic oscillator. Cavity QED enables high fidelity qubit state readout, cavity‐mediated two‐qubit gates, and storing quantum information in noise‐insensitive photonic states. In typical experiments a single mode of the cavity is utilized, however, coupling a qubit to multiple cavity modes – multimode cavity QED – realizes a number of new possibilities. One such possibility, which I will discuss in the main part of my talk, is the ability to engineer qubit‐qubit interactions. To demonstrate engineered interactions in a multimode system, we constructed a device which couples two flux‐tunable superconducting transmon‐type qubits using a planar three‐mode (three‐cavity) quantum filter. This multimode architecture allows for high contrast two‐qubit gates. On‐resonance the qubit‐qubit interactions are strong and we spectroscopically observe multimode strong couplings up to 102MHz. Conversely, off‐resonance the interactions are exponentially suppressed in the number of modes; for our three‐mode device the interactions are only 10kHz when the qubits are approximately 600MHz detuned from the cavity resonance. To validate the gate contrast, we utilize the multimode interaction to prepare a Bell state with >90% fidelity in 100ns. Multimode cavity QED is also an ideal platform to realize a system of photonic qubits, i.e., each mode of the cavity functioning as a two level system (zero or one photon in the mode). In the final part of the talk I will discuss our more recent work towards developing photonic qubits by utilizing the intrinsic multimode nature of 3D microwave cavities. To perform single‐ and two‐qubit gates between photonic qubits, each of the cavity modes are coupled to a single flux‐tunable JJ qubit. I will discuss some of the challenges involved and update our current progress.