High-frequency transmon readout free from multi-excitation resonances

The measurement of quantum bits is an essential part of quantum error correction protocols. The successful implementation of these protocols imposes stringent requirements on the measurement. First, the measurement should faithfully project the system onto an eigenstate of the observed quantity. Second, the experimenter must be able to reliably identify which eigenstate the qubit has been projected onto. A measurement that satisfies these criteria is called quantum non-demolition. In transmon qubits, readout is conventionally performed by applying a readout tone with a frequency close to that of the transmon. In this "dispersive" approach, information about the qubit state is encoded into the elastically scattered microwaves. So far, the non-demolition fidelity of dispersive measurement has been limited by the presence of undesired nonlinear resonances between the computational and non-computational transmon states. A high-power readout tone activates these multi-excitation resonances and spoils the non-demolition character of the measurement. Here, we find that strongly detuning the readout from the transmon frequency exponentially suppresses the strength of multi-excitation resonances. By using a readout frequency twelve times higher than the transmon frequency, we achieve a quantum non-demolition measurement fidelity exceeding 99.9%, bringing the readout performance on par with that of single- and two-qubit gates.