  Quantum error correction with erasure qubits promises significant advantages
over standard error correction due to favorable thresholds for erasure errors.
To realize this advantage in practice requires a qubit for which nearly all
errors are such erasure errors, and the ability to check for erasure errors
without dephasing the qubit. We demonstrate that a "dual-rail qubit" consisting
of a pair of resonantly coupled transmons can form a highly coherent erasure
qubit, where transmon $T_1$ errors are converted into erasure errors and
residual dephasing is strongly suppressed, leading to millisecond-scale
coherence within the qubit subspace. We show that single-qubit gates are
limited primarily by erasure errors, with erasure probability $p_\text{erasure}
= 2.19(2)\times 10^{-3}$ per gate while the residual errors are $\sim 40$ times
lower. We further demonstrate mid-circuit detection of erasure errors while
introducing $< 0.1\%$ dephasing error per check. Finally, we show that the
suppression of transmon noise allows this dual-rail qubit to preserve high
coherence over a broad tunable operating range, offering an improved capacity
to avoid frequency collisions. This work establishes transmon-based dual-rail
qubits as an attractive building block for hardware-efficient quantum error
correction.
