A Variable Coupling Network for Superconducting Qubits

We use a tunable resonator to vary the coupling between two fixed-frequency transmon qubits (4.6 and 5.0 GHz) mounted in a single 3-D superconducting cavity. The effective qubit-qubit coupling is provided by a thin-film lumped element resonator containing two single-junction SQUID loops, which is fabricated on the same chip as the qubits. An external flux, applied to the loops by an inductive drive coil, modulates the resonance frequency of the coupling circuit by nearly 1 GHz. The resonator capacitively couples to each transmon with a strength g ~ 50 MHz. When the resonator frequency is tuned from 4.6 to 4.2 GHz, the qubit-qubit coupling decreases by an order of magnitude, providing an effective switch between the devices.