High-fidelity state preparation and measurement protocol using multiple qubit encodings

We present a qubit state preparation and measurement (SPAM) protocol based on in-sequence error detection, that is universal to most qubit platforms. The only requirements for its implementation are the existence of a long-lived (metastable) level in the quantum system and a method for detection of population outside of this level that does not couple to it. We demonstrate the use of the protocol for three different qubit encodings in ¹³⁷Ba⁺: a ground-level (G) qubit, a metastable-level (M) qubit, and an optical (O) qubit. For all three qubit types we achieve extremely low SPAM infidelities of O: 7x10^-6, M: 5x10^-6, and G: 8x10^-6.