Fast Multiplexed Readout of Xmon Qubits Part II: Results

Fast and scalable qubit readout is an essential part of building a surface code based quantum computer. Here we show single- and multi-qubit frequency multiplexed readout of Xmon qubits with independent readout resonators coupled to a single readout line. We analyze both the CW behavior and the the transient response, finding that the ring-up time of the resonators is a major contribution to total readout time -- an important criterion for scalability in a fault tolerant system. Our bandpass filter design allows fast ring-up without compromising T1. We show single-qubit readout with an intrinsic fidelity of 99\% in 120~ns. Multiple-qubit readout is limited by amplifier saturation and achieves 99\% fidelity on 4 qubits in 200~ns. Correlated errors are a major problem for surface code quantum computing. We measure very low correlated errors and measurement crosstalk, which we attribute to using independent readout resonators.