Physicists have demonstrated all the components of an atomic clock - devices that measure time by measuring tiny energy shifts within an atomic nucleus. Such clocks could lead to significant improvements in precision measurements as well as new insights in fundamental physics.

The researchers measured the frequency of the light that causes the nuclei of the rare isotope thorium-229 to transition to a higher energy state - the "clock" of the atomic clock - with an accuracy 100,000 times higher than the previous best value. They achieved this by synchronizing the energy migration with the clock of the world's most precise clock. The work was led by Jun Ye at JILA, a research institute in Boulder, Colorado, and published September 5 in Nature. “It's really one of the most exciting papers in recent memory,” says Marianna Safronova, a nuclear physicist at the University of Delaware in Newark.

The breakthrough came by examining thorium-229 nuclei with a laser device called a frequency comb. The setup is not technically a watch as it has not yet been used to measure time. But such impressive results make the development of an atomic clock possible, says Safronova.

Measurements of the clock are already proving useful in particle physics, says Elina Fuchs, a theoretical physicist at Leibniz University Hannover, Germany. And since the clock's frequency is determined by the fundamental forces that hold the nucleus together, the prototype could determine whether a type of dark matter - an invisible substance that makes up about 85% of the matter in the universe - is influencing these forces on a tiny scale. “This is a new, direct window into nuclear power,” says Fuchs.

Ultimate timepieces

The world's best clocks, called atomic clocks, measure time using lasers - the frequency of light is precisely tuned to achieve the energy needed to move electrons between two energy levels within an atom. The most accurate atomic clock gains or loses only one second every 40 billion years. An atomic clock would work slightly differently: the clock would correspond to the energy transitions of protons and neutrons, rather than electrons, as they enter an excited state.

This energy shift requires a slightly higher, ultraviolet frequency, resulting in faster timing that could match or exceed the accuracy of the atomic clock. But the greatest potential advantage of the atomic clock lies in its combination of precision and stability. Particles in the nucleus are less sensitive than electrons to disturbances such as electromagnetic fields - meaning an atomic clock could be portable and robust. “It becomes desensitized in a way that is hard to imagine in terms of how our clocks work today,” says Anne Curtis, an experimental physicist at the National Physical Laboratory in Teddington, United Kingdom.

But finding the right type of atomic nucleus to use and determining the frequency needed to shift it to a different energy state has been a 50-year slog for physicists. In the 1970s, indirect evidence suggested that thorium-229 had a strangely low-energy nuclear transition - one that might eventually be triggered by tabletop plasma. But it wasn't until last year that scientists discovered the frequency needed - and this year they successfully initiated the transition with a laser.

The JILA team searched for the transition frequency in trillions of thorium-229 atoms embedded in the crystal using a system known as a frequency comb. The comb creates a series of laser frequency lines that are regularly and evenly spaced. This allows researchers to illuminate the crystal at many precise frequencies at once to search for a hit, rather than laboriously scanning through the spectrum of possible options with a single-frequency laser.

The comb's settings - including the width of the gaps between the lines, or "teeth" - were calibrated using the atomic clock and could be adjusted. The team conducted several experimental runs, and as they observed the characteristic glow that occurs when thorium-229 atoms decay from their excited state, they used the settings to calculate the frequency that controls the signal.

Observing the transition for the first time “felt amazing,” says study co-author Chuankun Zhang, a physicist at JILA. “We did tests all night long to check whether this was indeed the signal we were looking for,” he says.

Basic forces

What's special about the frequency comb is that it allows physicists to measure the frequency clock of a clock - here the thorium-229 core - as a ratio to another known frequency, in this case an atomic clock. This not only allowed the team to determine the absolute frequency value with high precision, but also opened up some interesting possibilities in physics, says Zhang.

If the speed of one clock's clock changes over time relative to another, it could indicate that factors that determine energy levels - such as the strong nuclear or electromagnetic force - are drifting or fluctuating, says Fuchs. Certain 'light' forms of dark matter, which have extremely low mass, are thought to have this effect, she says.

Any change in forces would be amplified in the frequency of core inward migration, so atomic clocks could potentially be about 100 million times more sensitive to the effects of this type of dark matter than atomic clocks. The latest result - which pinpoints the frequency to an accuracy of 13 decimal places - is already precise enough to narrow down the possible energy ranges in which light dark matter could exist, says Fuchs. Nuclear physics could also benefit from the more precise transition frequency, which could help scientists distinguish between different possible forms of the thorium-229 nucleus.

But more work needs to be done before atomic clocks can surpass atomic clocks - which are currently accurate to 19 decimal places. Researchers will study whether it makes sense to keep thorium-229 embedded in a crystal - a solid is handy for making a wearable watch - or whether confining individual atoms would produce better results.

The laser system also needs to be optimized. “Fortunately, this amazing technique has great potential,” says Olga Kocharovskaya, a physicist at Texas A&M University in College Station. It is a “prototype of the source to be used in the future watch,” she adds.