Barium-133 as a trapped-ion qubit

Short answer: Barium-133 is a radioactive barium isotope with a nuclear spin of one half and mostly visible-wavelength optical transitions, which makes it a candidate trapped-ion qubit. A 2017 Physical Review Letters paper trapped and laser cooled it and measured the spectroscopy needed, a 2020 npj Quantum Information paper reports a state preparation and measurement fidelity of 0.99971, and a 2026 conference abstract reports fast, high-fidelity qubit control. These are laboratory results, and the conference abstract is not a peer-reviewed paper.

Why barium-133

The 2017 abstract says barium-133 has been identified as an attractive ion for quantum information processing because of the combination of its spin-one-half nucleus and visible wavelength electronic transitions. Using a microgram source of radioactive material, the authors trap and laser cool this synthetic isotope in a radio-frequency ion trap. On the same single trapped atom, they measure isotope shifts and hyperfine structure of two electronic transitions needed for laser cooling, state preparation and state detection of the clock-state hyperfine and optical qubits. They also report an isotope shift for the rare barium 130 and 132 nuclides, which they say completes the spectroscopy needed to laser cool all long-lived barium II isotopes.

2020: measuring the missing energy levels

The npj Quantum Information abstract says the demonstrated trapping and cooling of barium-133 opened the door to using this nearly ideal atom for quantum information, but that unknown state energies were needed before high-fidelity operations. The authors measure hyperfine splittings of the P 3/2 and D 5/2 states and the transition frequencies connecting them. With these, they demonstrate high-fidelity hyperfine qubit manipulation with electron shelving detection to benchmark state preparation and measurement. Using single-shot threshold discrimination, they measure an average SPAM fidelity of 0.99971 with an uncertainty of 0.00003, which they describe as a factor of about two improvement over the best reported performance of any qubit.

2026: fast control and readout

A 2026 APS meeting abstract describes fast, high-fidelity state preparation and measurement of a single barium-133 ion. The level structure allows hyperfine and optical qubits within the same ion, and the ion can be loaded into a Paul trap by visible-light photoionization. The authors coherently manipulate the hyperfine qubit with microwave-driven transitions, and read out by mapping the qubit state onto a metastable D 5/2 manifold with 455 nm shelving and detecting fluorescence. They say off-resonant excitation from the shelving laser limits readout fidelity, and that additional 1762 nm optical transfer pulses address different hyperfine levels and significantly improve state measurement. Composite pulses suppress amplitude and detuning errors. They describe the platform as promising and mention future entangling-gate experiments with integrated photonics, which they are testing. This is a conference abstract, so the numbers are not repeated here.

Limits

These are single-ion experiments. Entangling gates are described only as future work in the 2026 abstract. A single-ion SPAM fidelity does not by itself show multi-qubit performance. The isotope is radioactive and produced from a small source, which the 2017 abstract describes in microgram quantities.

FAQ

Why is barium-133 interesting?

A spin-one-half nucleus plus visible-wavelength transitions, per the 2017 abstract.

How good was the readout?

An average SPAM fidelity of 0.99971 in the 2020 paper, as reported by the authors.

Is it radioactive?

Yes. It is a synthetic radioisotope, trapped from a microgram source.

What is next?

Entangling-gate experiments, which the 2026 abstract describes as planned.

Has it been used for a quantum computer?

No. The abstracts report single-ion control.