Mitigating qubit leakage errors in quantum circuits with gadgets and post-selection
Karl Mayer · 2022 IEEE International Conference on Quantum Computing and Engineering (QCE) · 2022
Leakage is a quantum error process in which a qubit ends up in a state outside of the computational subspace. In trapped-ion hyperfine qubits, leakage can occur as a result of spontaneous emission from an excited state during a two-qubit gate. This error can be detected by a circuit gadget that couples a data qubit to an ancilla qubit, followed by a measurement of the ancilla. The effects of leakage errors on the circuit can then be removed by post-selecting shots in which leakage was not detected. We first experimentally demonstrate improved two-qubit gate fidelity as measured by randomized benchmarking and cycle benchmarking. We then benchmark the use of leakage detection on system-level mirror benchmarking circuits. Finally, we present a protocol to measure the failure rate of the leakage detection gadget, defined as the probability that the gadget produces an error on the data qubit while not heralding the ancilla.