Takeaki Miyamura, Zhiling Wang, Peter A. Spring, Hiroto Mukai, Kohei Matsuura, Jesper Ilves, Yoshiki Sunada, Yasunobu Nakamura

Leakage out of the computational subspace is a major error mechanism in superconducting quantum processors. Detecting leakage without disturbing the encoded quantum information can provide a heralded error signal that error-correction decoders exploit. However, standard dispersive readout collapses all qubit eigenstates indiscriminately. Here, we demonstrate heralded leakage detection on a fixed-frequency transmon by applying a Rabi drive on the computational transition during dispersive readout. The drive makes the computational states indistinguishable to the probe on the resonator while leaving the second and higher excited states distinguishable from the computational states. We achieve a leakage-detection fidelity of 97.1(3)% within the 80-ns detection window, with a false-flag rate of 2.3(3)% from the computational subspace. Conditioned on the no-leakage outcome, the post-detection state retains an average fidelity of 92.9(5)% across the six cardinal states for an equal mixture of computational and leakage population. The scheme requires no circuit elements beyond those used for standard dispersive readout, making it applicable to various types of superconducting qubits without hardware modification.