Two research teams published the first working nuclear clocks in the journal Nature on Oct. 7. Each clock locks a laser to the energy jump inside a thorium-229 nucleus, a transition that ticks about 2 quadrillion times a second, roughly 220,000 times faster than the cesium atoms that define the second.
One team works at TU Wien in Austria. The other is led by Shiqian Ding in Beijing. Their papers, one from the Vienna group andone from the Beijing group, appeared the same day.
What a Nuclear Clock Does Differently From an Atomic Clock
Every clock counts the oscillations of something that repeats at a steady rate. Atomic clocks count the light that an atom’s electrons absorb when they jump between energy levels. The definition of the second is tied to a cesium-133 atom and the number 9,192,631,770, the number of cycles of its ground-state transition per second.
A nuclear clock counts a jump inside the nucleus instead. Nuclei almost never have a transition a laser can reach, because nuclear energy steps are usually thousands of times larger than the energy in a laser photon. Thorium-229 is the exception. The Vienna paper says the nucleus has an excited state about 8 electronvolts above its ground state, and attributes that small gap to a “coincidental near-cancellation” of two very large nuclear energy contributions.
An 8 electronvolt gap sits in the vacuum ultraviolet, at a wavelength near 148 nanometers. Light that short is absorbed by air, so both teams had to build laser systems that work in that range. The Beijing paper reports a laser generated by mixing light in cadmium vapor, producing 10 microwatts, about 5 microwatts of which reached the crystal.
Both clocks hold the thorium inside a calcium fluoride crystal at roughly room temperature. The Vienna crystal is 3.1 millimeters across and 4.2 millimeters long. Nuclei in a solid do not need the vacuum chambers and cooled atoms that conventional atomic clocks need, which is why researchers see a path to smaller and sturdier timekeepers.
What the Nature Papers Measured
The step that makes this a clock, and not a spectroscopy experiment, is a feedback loop. A laser can hit the thorium transition once and show it exists. A clock has to read the nuclear signal continuously and correct the laser to keep it on the line. Both papers report that loop.
The Vienna team, with corresponding author Thorsten Schumm, locked its laser to the transition and compared it against a ytterbium ion clock for about 23 hours of data. The paper reports a fractional instability of 3 parts in a trillion over one second, approaching 1 part in a quadrillion over a day. Over a day, the clock’s drift was (2 ± 4) parts in 100 trillion, consistent with zero.
The Beijing paper, whose last-listed author is Shiqian Ding, reports an instability of 5 parts in 10 trillion over one second. It also compares two separately made crystals. Their clock frequencies differed by 560 hertz out of about 2 quadrillion, a fractional difference of about 2.8 parts in 10 trillion. The Beijing paper also says the crystals’ frequencies agree with an earlier measurement from the JILA institute in Colorado to within the uncertainty of the new data.
How Long It Takes to Reach the Best Atomic Clocks, By the Numbers
Neither clock beats the best atomic clocks yet. Simple arithmetic on the Beijing figures shows how far apart the numbers are. Instability of this kind falls with the square root of averaging time. Starting at 5 parts in 10 trillion at one second, reaching 1 part in a quadrillion requires averaging for 250,000 seconds, about 2.9 days, if the noise behaves the same way throughout. The Vienna paper projects an instability of about 1 part in a quadrillion at one second with longer crystals and about 1 microwatt of laser power, and labels that a projection and not a measurement.
Another number shows how little thorium the technique uses. The Beijing paper says one of its crystals was grown from a solution containing 1.4 micrograms of thorium-229. The amount is small enough that the crystal’s thorium measures about 10 kilobecquerels of radioactivity.
Reaction From Physicists and the Dark Matter Test
Akio Kawasaki of Japan’s National Metrology Institute wrote the accompanying commentary in Nature. He said the papers report the first nuclear clock implementations, calling them “a milestone in the field of metrology.”
Science News reported both teams’ results in June, when they appeared as preprints that had not been peer reviewed. Schumm told the magazine in its own interview that “in some types of measurements, we’re already outperforming all of the atomic clocks.” The Oct. 7 publication follows peer review and adds the dark matter analysis from Vienna.
That analysis used the clock as a detector. If ultralight dark matter exists and couples to ordinary matter, it could make the thorium transition frequency wobble or drift. The Vienna team watched for periodic changes on timescales from 20 seconds to a day. They found no signal above their 5% detection threshold and set upper limits instead. The paper says those limits compete with the best atomic clocks for dark matter coupling to light and go beyond earlier results for coupling to the strong force.
What happens Next for Nuclear Clocks
The Vienna authors measured the transition’s sensitivity to temperature at 1.8 kilohertz per kelvin near room temperature, with line-center shifts up to 1.7 kilohertz depending on where the laser struck the crystal. Those effects are among the practical limits on how stable the first clocks can be. The Vienna team also measured a linewidth near 100 kilohertz.
The Beijing team resolved a narrower 27 kilohertz line, and the Beijing paper reports an excited-state lifetime of 631 seconds, which sets how narrow the transition can eventually get.
Researchers have worked toward this since a thorium nuclear clock was first proposed in 2003, according to Science News. The two Nature papers do not claim a new time standard. They show that the nucleus can serve as the pendulum, that a laser can be held on it continuously, and that the resulting signal is reproducible between crystals made by different groups.
