Implementation of a generalized controlled-NOT gate between fixed-frequency transmons
Shavindra Premaratne, Jen-Hao Yeh, F. C. Wellstood, B. S. Palmer · Physical Review A · 2019
We have embedded two fixed-frequency ${\mathrm{Al}/\mathrm{AlO}}_{x}$/Al transmons, with ground-to-excited transition frequencies at 6.0714 and 6.7543 GHz, in a single three-dimensional Al cavity with a fundamental mode at 7.7463 GHz. Strong coupling between the cavity and each transmon results in an effective qubit-qubit coupling strength of 26 MHz and a $\ensuremath{-}1$ MHz dispersive shift in each qubit's transition frequency, depending on the state of the other qubit. Using the all-microwave SWIPHT (speeding up wave forms by inducing phases to harmful transitions) technique [Economou and Barnes, Phys. Rev. B 91, 161405 (2015)], we demonstrate the operation of a generalized controlled-not gate between the two qubits, with a gate time of ${\ensuremath{\tau}}_{g}=907$ ns optimized for this device. Using quantum process tomography we find that the gate fidelity is 83--84%, somewhat less than the 87% fidelity expected from relaxation and dephasing in the transmons during the gate time.