UCLA

Building a fault-tolerant quantum computer requires more than improving physical hardware in isolation. Hardware-level design choices must be guided by the architecture of the full quantum computer and by the bottlenecks that ultimately limit its performance. The 133Ba+ ion is well suited for this kind of architecture-driven optimization. Its simple nuclear spin structure, with I = 1/2, enables fast, high-fidelity state preparation and measurement (SPAM). We leverage this advantage to demonstrate record SPAM fidelity, along with a state-preparation time more than three orders of magnitude faster than previous record-level SPAM demonstrations. This makes 133Ba+ especially promising as a reusable ancilla qubit for quantum error correction.The long-lived D5/2 state, with τ = 30 s, also makes 133Ba+ uniquely positioned to take advantage of the optical-metastable-ground (OMG) qubit protocol, which enables same-species sympathetic cooling. Within this protocol, we demonstrate a magic polarization condition that allows global gates with 532 nm light to be applied in one qubit subspace while leaving other subspaces largely unaffected. We also demonstrate stimulated Raman gates in both the ground and metastable qubit subspaces with errors below the fault-tolerance threshold, supported by a direct measurement of the spontaneous Raman scattering rate. The main practical drawback of 133Ba+ is its radioactivity. Here, we mitigate this challenge by demonstrating ion loading from a 133Ba target below the Nuclear Regulatory Commission exempt quantity of 10 µCi. This is enabled by a new autoionization loading scheme that substantially reduces loading times for barium ions.