Characterizing Control of a High Fidelity Beamsplitter in the Single-Photon Subspace

Recent demonstrations of highly coherent beamsplitting interactions between two high-Q bosonic modes in circuit QED platforms motivate us to characterize our control of this interaction under pulsed operation. To do this, we work in the joint single photon subspace of the two-cavity system ({|01>, |10>}), which can be mapped onto a Bloch Sphere. Here, the beamsplitter interaction generates arbitrary rotations, allowing us to apply conventional qubit control characterization techniques to this system. After tuning up beamsplitter gates in the presence of drive-induced frequency shifts, we run a randomized benchmarking protocol, achieving an average gate fidelity of F > 99.90%. Further, we show that the fidelity is mostly limited by cavity decay, which suggests an ultimate fidelity approaching 99.99% should be possible.