Calibrating two-qubit gates via robust phase estimation

Robust phase estimation (RPE) is a Heisenberg-limited characterization protocol for learning the phases of quantum gate operations. Unlike many other similar techniques, RPE requires neither ancillae qubits nor highly accurate state preparation and measurement. Additionally, the computational requirements of RPE are minimal, needing nothing more complex than inverse trigonometric functions. To date, RPE has been used for single-qubit gate characterization and calibration. We extend RPE for two qubits, showing that it may be used to learn the phase for any standard two-qubit gate (e.g., CNOT, CPHASE, Molmer-Sorensen, SWAP, cross-resonance, etc.). We provide both numerical and experimental results.