Sequential state preparation and measurement of multiple qubits via a single channel

Accurate state preparation and measurement (SPAM) is vital for performing high-fidelity quantum computation 1–3 . However, systems typically use individual readout hardware for each qubit, presenting a significant overhead to the scale-up of quantum computers 2,4 . In response, frequency-multiplexed simultaneous quantum non-demolition (QND) readout through a single channel has been investigated 5–11 . In this work, we investigate an alternative solution: time-multiplexed QND readout of multiple data qubits through a single readout channel. We present Bayesian estimation scheduling procedures, enabling initialisation and readout with fidelities comparable to separate readout channels. We show that up to four qubits can be measured and initialised using a single channel with fidelities >99.9% and >99.95% respectively, even when ancilla single-shot fidelities are ~95%. These results show that minimising data qubit flips enables high-fidelity SPAM for sequential QND readout of multiple qubits, even with poor single-shot readout, using only a single shared readout channel.