Improving Multilevel Qudit Readout Fidelity During Relaxation Events via Hidden Markov Models*

Quantum state determination with high fidelity is a requirement for quantum computation. However, qubit relaxation imposes a limiting constraint on readout fidelity. Longer readout measurement times increase the distinguishability between the qudit states, however state decay during a long readout pulse can obfuscate the measurement and result in misclassification of qudit state. To overcome this constraint, we demonstrate high-fidelity multi-state readout by detecting qubit relaxation with Hidden Markov Models on LLNL’s Quantum Design and Integration Testbed (QuDIT). The ability of Hidden Markov Models to account for relaxation and thermal excitation processes allows for longer readout times, extraction of transition probabilities, and higher readout fidelity.