The Toffoli gate is a three-qubit operation that inverts the state of a target qubit conditioned on the state of two control qubits. It enables universal reversible classical computation, forms a universal set of gates in quantum computation together with a Hadamard gate and is also a key element in quantum error correction schemes. Here, Andreas Wallraff and colleagues at ETH-Zurich demonstrated the implementation of a Toffoli gate with three superconducting transmon qubits coupled to a microwave resonator. By exploiting the third energy level of the transmon qubit, the number of elementary gates needed for the implementation of the Toffoli gate has been greatly reduced in comparison to theoretical proposals using two-level systems only. The gate fidelity evaluated with use of full process tomography and Monte Carlo process certification is found to be 68.5±0.5%. The results reinforce the potential of macroscopic superconducting qubits for implementation of complex quantum operations and point at the possibility to implement quantum error correction schemes.
A. Fedorov, L. Steffen, M. Baur, M. P. da Silva, and A. Wallraff, Nature 481, 170–172 (2012)