High-fidelity two qubit gate via single flux quantum pulses

We analyze two-qubit gates controlled by trains of single flux quantum (SFQ) pulses. The SFQ pulses are applied to one of two weakly coupled superconducting qubits. We analyze a controlled Z gate realized by a resonant transition from state $|11\rangle$ to a higher energy state outside of the qubit computational space and returning back to $|11\rangle$. We evaluate the fidelity of this two-qubit gate and find that the fidelity can be above 99\% for gate times under 100 ns. For systems with fixed coupling between qubits, the fidelity of single qubit gates can also exceed 99\%, and can be even higher if tunable coupling is used. We also discuss alternative routes for two-qubit gates utilizing single flux pulses.