  A challenge for constructing large circuits of superconducting qubits is to
balance addressability, coherence and coupling strength. High coherence can be
attained by building circuits from fixed-frequency qubits, however, leading
techniques cannot couple qubits that are far detuned. Here we introduce a
method based on a tunable bus which allows for the coupling of two
fixed-frequency qubits even at large detunings. By parametrically oscillating
the bus at the qubit-qubit detuning we enable a resonant exchange (XX+YY)
interaction. We use this interaction to implement a 183ns two-qubit iSWAP gate
between qubits separated in frequency by 854MHz with a measured average
fidelity of 0.9823(4) from interleaved randomized benchmarking. This gate may
be an enabling technology for surface code circuits and for analog quantum
simulation.
