Coherent storage of quantum information is crucial to many quantum technologies. Long coherence times have been demonstrated in trapped-ion qubits, typically using the hyperfine levels within the ground state of a single ion. However, recent research suggests qubits encoded in metastable states could provide architectural benefits for quantum information processing, such as the possibility of effective dual-species operation in a single-species system and erasure-error conversion for fault-tolerant quantum computing. Here, we demonstrate long-lived encoding of a quantum state in the metastable states of a trapped ion. By sympathetically cooling with another ion of the same species, employing a tuned CPMG dynamical-decoupling sequence, and constantly monitoring for leakage errors, we demonstrate a coherence time of 136(42) s with a qubit encoded in the metastable $5D_{5/2}$ state of a single ${}^{137}{\mathrm{Ba}}^{+}$ ion. In agreement with a model based on empirical results from dynamical-decoupling-based noise spectroscopy, we find that dephasing of the metastable levels is the dominant source of error once leakage errors are removed.

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• Quantum coherence & coherence measures
• Quantum information architectures & platforms
• Quantum information processing
• Quantum information with trapped ions
• Quantum memories
• Trapped ions