High-fidelity heralded quantum state preparation and measurement

TL;DR

This paper introduces a new protocol for high-fidelity qubit state preparation and measurement that reduces errors significantly, applicable to various quantum systems with metastable states, demonstrated on a trapped ion with record low infidelities.

Contribution

A novel SPAM protocol combining standard methods with in-sequence error detection, achieving unprecedented fidelity across multiple qubit encodings.

Findings

Achieved lowest reported SPAM infidelities for optical, metastable, and ground-level qubits.

Demonstrated protocol applicability to different qubit encodings in a single ion.

Reduced error rates to below 10^-5 in all tested cases.

Abstract

We present a novel protocol for high-fidelity qubit state preparation and measurement (SPAM) that combines standard SPAM methods with a series of in-sequence measurements to detect and remove errors. The protocol can be applied in any quantum system with a long-lived (metastable) level and a means to detect population outside of this level without coupling to it. We demonstrate the use of the protocol for three different qubit encodings in a single trapped $^{137}\mathrm{Ba}^+$ ion. For all three, we achieve the lowest reported SPAM infidelities of $7(4) \times 10^{-6}$ (optical qubit), $5(4) \times 10^{-6}$ (metastable-level qubit), and $8(4) \times 10^{-6}$ (ground-level qubit).