Synopsis: Radioactive Qubits

The trapping and cooling of the radioactive isotope barium-133 offers up an attractive system for encoding quantum information.
Synopsis figure
D. Hucul/University of California, Los Angeles

Trapped and laser-cooled ions are a popular choice to serve as quantum bits. Ideally, these ions should have an internal structure that allows robust manipulation of the ions’ states, as well as laser-cooling transitions at visible wavelengths. Only the barium-133 ion satisfies both these criteria. The one hitch is that barium-133 is radioactive, with a half-life of 10.5 years, which means it isn’t naturally occurring. Researchers from the University of California, Los Angeles, have managed for the first time to trap and cool a single synthetically produced barium-133 ion. The team also measured the ion’s spectrum, the details of which are needed for preparing and manipulating the qubit states of these ions.

Barium-133 is an alkaline-earth element, so its ionized state has a single outer-shell electron with easy-to-use hydrogen-like states. Another convenient feature of its internal structure is a nuclear spin of 1/2, which allows it to have so-called “clock states” that are stable against noise induced by magnetic fields. In addition, its cooling transitions allow the use of visible wavelength lasers and other optics devices.

To take advantage of barium-133’s ideal properties, David Hucul and colleagues developed a trapping and cooling method for the ion. They started with a solid sample artificially enriched with 2% barium-133 and applied laser ablation to load an ion trap with up to 100 barium ions. Because of its low concentration, only about one of these ions was barium-133; most of the other ions were barium-132. Fortunately, the laser beams used to cool barium-133 simultaneously heated barium-132 and other unwanted isotopes, causing them to escape the trap. Following this isotope purification, the researchers were able to precisely measure the spectrum of a single barium-133 ion.

This research is published in Physical Review Letters.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.


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