Nuclear clocks could surpass atomic clocks as the most accurate watches

Nuclear clocks could be the GOAT: the largest of all watches.

If physicists could build them, nuclear clocks would be a whole new type of clock, which would tell time based on the physics of the core of atoms. Today’s most accurate clocks, called atomic clocks, rely on the behavior of the electrons in atoms. But a clock based on atomic nuclei could achieve 10 times the accuracy of these atomic clocks, the researchers estimate.

Better clocks could improve the technologies that depend on them, like gps navigation, physicist Peter Thirolf said on June 3 at an online meeting of the Division of Atomic, Molecular and Optical Physics of the American Physical Society. But “it’s not just about timing. Unlike the electrons of atoms, atomic nuclei are subject to strong nuclear force, which holds protons and neutrons together. “A nuclear clock sees another part of the world,” said Thirolf, of Ludwig-Maximilians-Universität München in Germany. This means that nuclear clocks could allow new tests of fundamental ideas in physics, including whether supposedly immutable numbers in physics known as fundamental constants are, in fact, constant.

Atomic clocks keep track of time using the energy jumps of the electrons in atoms. According to quantum physics, electrons in atoms can only carry certain amounts of energy, at specific energy levels. To move the electrons of an atom from one energy level to another, the atoms in an atomic clock must be struck with laser light of the correct frequency. This frequency – the rate of oscillation of the electromagnetic waves of light – serves as a very precise timekeeper.

Like electrons in an atom, protons and neutrons in atomic nuclei also occupy discrete energy levels. Nuclear clocks are said to be based on jumps between these levels of nuclear energy, rather than those of electrons. In particular, the nuclei are resistant to the effects of parasitic electric or magnetic fields which can hamper atomic clocks. As a result, nuclear clocks “would be more stable and more precise,” explains theoretical physicist Adriana Pálffy of Friedrich-Alexander-Universität Erlangen-Nürnberg in Germany.

But there is a problem. To keep track of time with nuclei, scientists must be able to trigger the jump between nuclear energy levels with a laser. “Nuclear levels are not normally accessible with lasers,” theoretical physicist Marianna Safronova of the University of Delaware said at a June 2 conference at the meeting. For most nuclei, this would require light of more energy than the proper lasers can reach. Fortunately there is only one exception in all known nuclei, said Safronova, “a bizarre thing of nature”. A variety of thorium called thorium-229 has a pair of energy levels close enough in energy that a laser could potentially trigger the jump.

Recent measurements have made it possible to locate more precisely the energy of this jump, a crucial step towards the construction of a thorium nuclear clock. Thirolf and his colleagues estimated energy by measuring the electrons emitted when the nucleus jumps between the two levels, as shown in Nature in 2019. And in an article from 2020 in Physical examination letters, physicist Andreas Fleischmann and his colleagues measured other energy jumps that the thorium nucleus can make, subtracting them from deduce energy from nuclear clock jump.

microscope image of detectors
An array of highly sensitive detectors (shown in a false-color scanning electron microscope image) measured the energy of light emitted when thorium-229 atoms jumped between energy levels. These measurements allowed Andreas Fleischmann and his colleagues to estimate the jump energy that physicists want to use to make a nuclear clock.Matthäus Krantz

The teams agree that the jump is just over 8 electron volts of energy. This energy corresponds to ultraviolet light in a range for which triggering the jump with a laser is possible, but at the limit of the capabilities of scientists.

Now that physicists know the size of the energy jump, they aim to trigger it with lasers. At the meeting, physicist Chuankun Zhang of the JILA Research Institute in Boulder, Colorado, reported efforts to use a frequency comb (NS: 10/05/18) – a method of creating an array of discrete frequencies of laser light – to initiate the jump and measure your energy even better. “If it’s successful, we can directly build a nuclear-based optical clock from it,” he said at the meeting. Thirolf’s team is also working with frequency combs, aiming for a functional nuclear clock within the next five years.

Meanwhile, Pálffy plans to use what is called an “electronic bridge”. Rather than using a laser to directly initiate an energy jump through the nucleus, the laser would first excite the electrons, which transfer energy to the nucleus, Pálffy reported at the meeting.

Nuclear clocks could allow researchers to design new tests to determine whether fundamental constants in nature vary over time. For example, some studies have suggested that the fine structure constant, a number that defines the strength of electromagnetic interactions, might change (NS: 11/2/16). “This nuclear clock is a perfect system to look for the variation of fundamental constants, ”said Victor Flambaum of the University of New South Wales in Sydney at the meeting. The devices could also test a foundation of Einstein’s general theory of relativity called the equivalence principle (NS: 12/04/17). Or they could search for dark matter, elusive, undetected particles that physicists believe make up most of the matter in the universe, which could change the ticking of the clock.

The potential of nuclear clocks is so promising that for Fleischmann, of the University of Heidelberg in Germany, it only took a moment to resolve the dilemma of how scientists could build a nuclear clock, he says. It was “from the first second clear that this is an issue to be worked on”.


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