Microwave-controlled universal entangling gate for fixed-frequency superconducting qubits

A basic requirement for fault tolerant quantum computing is a universal set of nearly perfect single- and two-qubit gates. As high-fidelity single-qubit operations on superconducting qubits become routine^1,2, the focus shifts onto developing robust and scaleable two-qubit gates. Already, rapid progress has been made, including a controlled-NOT gate with fixed coupled qubits³, and highly entangled states of two^4,5 and three^6,7 qubits generated from tuning the qubits to explicit resonance conditions. However, as superconducting systems scale up, fixed-coupled systems become difficult to individually address, and tuning to resonances in a spectrally crowded frequency space is problematic. Here we demonstrate a new microwave controlled two-qubit gate^8-10 on capacitively-shunted flux qubits¹¹ parked at locations of optimal coherence and coupled via a quantum bus. The effective interaction is tunable via irradiation of one qubit at them other qubit's transition frequency. We use this cross-resonance (CR) gate to generate Bell states with a maximal fidelity of 90% and concurrence of 0:88, limited by qubit relaxation. Quantum process tomography gives a gate fidelity of 81%. This gate scheme is readily extendable to systems of more than two qubits and would permit coupling of qubits which are non-nearest neighbours in frequency.