Quantum information processing with the metastable states of ¹³⁷Ba⁺

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) seconds with a qubit encoded in the metastable 5D_5/2 state of a single ¹³⁷Ba⁺ ion. Besides, we discuss our recent progress towards high-fidelity, universal control of a qudit encoded in the metastable states using multi-tone RF signals. Previous work with a transmon system [1] has shown that the multi-tone displacement operation combined with the SNAP gate results in universal control of a transmon qudit. We discuss our implementation of such a scheme with metastable states of a trapped ¹³⁷Ba⁺ ion and our progress toward implementing quantum algorithms.

[1] Champion, Elizabeth, et al. arXiv preprint arXiv:2405.15857 (2024).