Qubit Syndrome Measurements with a High-Fidelity Rb-Cs Rydberg Gate.

We demonstrate an interspecies entangling Rydberg gate between rubidium (Rb) and cesium (Cs) atoms with fidelity F=0.975±0.002. The two-species atom array enables in-place quantum nondemolition (QND) qubit measurements which are a key capability for quantum error correction. We demonstrate this functionality with multiatom error syndrome measurements achieving QND measurement fidelities of F_{QND}=0.933(12) and 0.865(17) for two- and three-qubit plaquettes, respectively.