Realization of a two-qubit gate using a coherently driven coupler

Fast and high-fidelity iSWAP and CZ gates can be realized using frequency-tunable transmons as nonlinear coupling elements between two superconducting qubits [1-4]. These gates are typically performed by flux-tuning the frequency of the coupling transmon which leads to a modification of the qubit-qubit coupling strength. In this work, we experimentally demonstrate that the interaction between two qubits can also be controlled by coherently driving the coupler. For a particular qubit frequency, there is no interaction when the coupler is in its ground state while high coupling rate is achieved when the coupler is excited. To perform an iSWAP gate in this configuration, we apply a resonant two-pi pulse on the coupler. The population transfer between qubits is maximal when the pulse duration matches the qubit coupling rate. Our coupling scheme could significantly reduce the complexity of superconducting quantum processors by eliminating the need for coupler flux tuning lines and the associated flux crosstalk and settling tails calibrations. 

[1] Li et al., Phys. Rev. Applied 14, 024070 (2020)

[2] Foxen et al., PRL 125 120504 (2020) 

[3] Collodo et al., PRL 125 240502 (2020) 

[4] Sung et al., PRX 11 021058 (2021)