A Tick-based Radionuclear Prototype for a Dark-Matter Detection Program at JILA

atomic clocks have lasers that match the energy it takes to move electrons between two levels inside an atom. The most accurate atomic clock gains or loses only one second every 40 billion years. A nuclear clock would work slightly differently: the tick would correspond to the energy transitions of protons and neutrons, rather than electrons, as they reshuffle into an excited state.

Finding this one needed to be done with an enormous range of possible values because theoretical models of the nucleus aren’t accurate in predicting energy transitions. Moreover, thorium-229 decays slowly from its first energy state. The chance of observing the decay is low with a half-life of around 30 minutes.

Atomic clocks currently hold the world record for most accurate timekeeping. They tick out time through the frequency of light emitted and absorbed by electrons jumping between atomic energy levels. Lasers that are locked to a clock’s frequency provide the read-out.

The tick is being used in particle physics, according to Elina Fuchs, a theoretical physicist. And because the clock’s frequency is set by the fundamental forces that hold together the nucleus, the prototype could spot whether a type of dark matter — an invisible substance that accounts for around 85% of material in the Universe — affects these forces on a minuscule scale. The nuclear force has a new, direct window onto it.

The width of the gaps between the lines, called teeth, can be adjusted using the atomic clock. The team conducted several experimental runs, and when they observed the tell-tale glow produced when thorium-229 atoms decay from their excited state, they used the settings to calculate the frequency driving the signal.

Observing the transition for the first time “felt amazing”, says study co-author Chuankun Zhang, a physicist at JILA. “We spent the entire night doing all the tests to check if this is actually really the signal that we were looking for,” he says.

The laser system also needs honing. Kocharovskaya is a physicist at Texas A&M University in College Station. She believes that the source will be used in the future clock.