Prototype crystal could make nuclear clocks up to 10 times more precise

For over 15 years, physicist Thorsten Schumm's team at the Vienna Institute of Technology worked on a crystal precise enough to host a nuclear clock. They have now found the best seat for a thorium-229 atom inside calcium fluoride, and used it to build the first prototype of a clock that could eventually fit on a small chip.

The challenge was not simply making a crystal. When thorium-229 occupied the wrong position in the crystal lattice, it created an uneven electric field that disturbed the clock's timing. The researchers tested four possible sites by shining an ultraviolet laser for 60 seconds, switching it off, and waiting five minutes while the nuclei emitted light as they returned to lower energy levels.

Three sites sent the light back at different wavelengths, a sign that the surrounding electric field was uneven. The fourth returned it at a single wavelength. That was the site the team selected for its nuclear-clock prototype. The findings were published in Science.

A conventional atomic clock measures electrons jumping between energy states. A nuclear clock goes deeper, measuring an energy change involving a neutron inside the nucleus. The electron cloud provides a natural shield from stray electric and magnetic fields, and a crystal-based design can work at room temperature, reducing the need for the large vacuum chambers and shielding used by existing clocks. The ultraviolet laser is a practical substitute for the gamma-ray laser that would otherwise be needed, since that gamma-ray laser has not yet been made.

So what changes, concretely? The researchers say the design could be up to 10 times more precise than existing clocks while becoming small enough for compact devices. The team expects precision to improve by at least three orders of magnitude by the end of the year, but the immediate focus is miniaturization. The prototype is still a research-stage device, not a deployed product.