Scientists have made a major breakthrough that takes us one step closer to developing a nuclear clock – a device that tells time based on the inner workings of atoms.
For the first time, physicists have used laser light to raise the nucleus of a thorium atom to a higher energy level. The discovery paves the way for the development of a new clock whose ticking is not only more precise, but can probe the most fundamental forces in the universe.
The researchers published their findings April 29 in the journal Physical examination letters.
“Seeing the first signal was a dream come true,” lead researcher Torsten Schumm, a professor of quantum metrology at the Vienna University of Technology, told Live Science. “[It’s] the reward for many years of preparation, while doubting whether it would actually work.”
It is time
Currently ours the most accurate clocks are atomic clocks and keep time by firing lasers at electrons—matching the laser’s frequency to the precise jumps through energy levels it causes electrons orbiting atoms to make. This method gives scientists an ultra-precise measurement of the laser’s frequency, from which they can extract the “ticks” of the atomic clock.
However, atomic clocks are far from perfect. The electrons they rely on to keep time are found outside atoms. They are therefore vulnerable to encroachment by homeless people magnetic fields or other environmental effects that may slightly alter their energy levels, the frequency of laser light to which they subsequently respond, and therefore the time they retain.
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A nuclear clock, on the other hand, would use the energy transitions of the nuclei at the heart of the atom, so they are protected from outside interference. But many of the gaps between the energy levels of nuclei are thousands of times larger than those for electrons – meaning they are too large to be crossed with the energy of a laser.
But in the 1970s, scientists discovered that an isotope, or version, of the element thorium (thorium-229) appeared to have an energy level that could be covered by laser light.
But finding that exact energy gap was no easy task. Initially, the researchers excited thorium-229 to an energy level far above the two that physicists are actually interested in. They then measure subtle differences in the energy of the emitted light as it falls back to the higher compared to that just below it.
The researchers compared this process to finding the height of a curb by dropping balls off a skyscraper—subtle differences in the bounce heights when the ball hits the street and when it hits the pavement can help them navigate the small distance between them.
Over the past 50 years, research has narrowed down the energy required to cause this energy level jump to small fractions of an electron volt — but this precision was still not enough.
“The theory tells us that it was somewhere in the energy range between 0 eV and 10 eV, but we have to hit the right frequency to within 7 to 8 digits to cause an effect,” Schumm said. “Scanning the entire search scope would take millennia, so we had to narrow down the search scope over many years of preparatory experiments.”
To finally pinpoint the exact value, Schumm and his team trapped about 10 to the power of 17 thorium-229 nuclei (or a million times more nuclei than there are stars in our galaxy) in calcium fluoride crystals, greatly increasing the likelihood of finding desired transition. After many attempts, the researchers directly observed a thorium atom jumping between energy levels: an energy change of 8.35574 electron volts.
The researchers note that it will take many more years to develop nuclear clocks with the same accuracy as their atomic counterparts. But with this transition finally spotted, the window is finally open and could allow physicists to probe more deeply into the elusive nature of dark energy, dark matter, and the fundamental forces of our universe.
“The nuclear clock will provide an extremely precise measurement of the energy difference between two bound states of the nucleus,” Schumm said. “These two binding energies are the result of three of the four fundamental forces in physics: electromagnetism, the strong nuclear force, and the weak nuclear force. This is in contrast to all atomic clocks that rely only on electromagnetism. If one of these three fundamental forces changes as a function of time or location in space, the nuclear clock should see that.”
