  The development of scalable, high-fidelity qubits is a key challenge in
quantum information science. Neutral atom qubits have progressed rapidly in
recent years, demonstrating programmable processors and quantum simulators with
scaling to hundreds of atoms. Exploring new atomic species, such as alkaline
earth atoms, or combining multiple species can provide new paths to improving
coherence, control and scalability. For example, for eventual application in
quantum error correction, it is advantageous to realize qubits with structured
error models, such as biased Pauli errors or conversion of errors into
detectable erasures. In this work, we demonstrate a new neutral atom qubit,
using the nuclear spin of a long-lived metastable state in ${}^{171}$Yb. The
long coherence time and fast excitation to the Rydberg state allow one- and
two-qubit gates with fidelities of 0.9990(1) and 0.980(1), respectively.
Importantly, a significant fraction of all gate errors result in decays out of
the qubit subspace, to the ground state. By performing fast, mid-circuit
detection of these errors, we convert them into erasure errors; during
detection, the induced error probability on qubits remaining in the
computational space is less than $10^{-5}$. This work establishes metastable
${}^{171}$Yb as a promising platform for realizing fault-tolerant quantum
computing.
