Driving two-qubit entanglement with an enhanced ZZ interaction in circuit QED*

The quantum bus architecture is fast becoming a popular approach for coupling superconducting qubits [1,2]. With two fixed-frequency qubits coupled by a resonator, it is possible to engineer the system's frequencies such that the qubits experience a strong ZZ interaction. This interaction can be used as a resource for creating entanglement when needed, but can also be suppressed at will using appropriate decoupling sequences. We will show measurements of a device where this ZZ interaction is enhanced by interactions with higher-levels of superconducting transmon qubits. To achieve high-fidelity control in this regime, we employ robust composite pulses and optimal control methods to decouple the two-qubit interaction during single-qubit operations. The resulting system serves as a testbed for adapting control techniques from liquid-state NMR to fixed-frequency superconducting qubits.\\[4pt] [1] L. DiCarlo {\it el al}. Nature {\bf 460}, 240-244 (2009).\\[0pt] [2] Matteo Mariantoni {\it et al}. Science {\bf 334}, 61-65 (2011).