Nuclear clocks could be the GOAT: the largest of all clocks.
If physicists could build them, nuclear clocks would be a new type of clock, which would keep time based on the physics of the heart of atoms. Today’s most accurate clocks, called atomic clocks, depend on the behavior of the electrons in the atoms. Researchers estimate that a clock based on atomic nuclei could reach 10 times the accuracy of those atomic clocks.
Better clocks could improve technologies that depend on them, such as GPS navigation, said physicist Peter Thirolf on June 3 during an online meeting of the American Physical Society's Division of Atomic, Molecular and Optical Physics. But "it's not just about timing." Unlike the electrons in atoms, atomic nuclei are subject to strong nuclear force, which holds protons and neutrons together. “A nuclear clock sees a different 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 the supposedly immutable numbers in physics known as fundamental constants are, in fact, constants.
Atomic clocks count time using the energy jumps of the electrons in the atoms. According to quantum physics, the electrons in atoms can only carry certain amounts of energy, at specific energy levels. To beat electrons in an atom from one energy level to another, the atoms of an atomic clock must be struck with a laser light of exact frequency. That frequency, the rate of oscillation of the electromagnetic waves of light, serves as a highly accurate timer.
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Like the electrons of an atom, the protons and neutrons of atomic nuclei also occupy discrete energy levels. Nuclear clocks would be based on jumps between those levels of nuclear energy, rather than those of electrons. In particular, nuclei are resistant to the effects of lost electric or magnetic fields that can hinder atomic clocks. As a result, nuclear clocks "would be more stable and accurate," says theoretical physicist Adriana Pálffy of Friedrich-Alexander-Universität Erlangen-Nürnberg in Germany.
But there is a problem. To count time with the nuclei, scientists need to be able to launch 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 in a June 2 talk at the meeting. For most cores, this would require more energy light than the right lasers can achieve. Fortunately, there is one exception in every known nucleus, Safronova said, "a freak thing of nature." A variety of thorium called thorium-229 has a couple of energy levels close enough in energy for a laser to trigger the jump.
Recent measurements have more accurately identified the energy of that jump, a crucial step in building a thorium nuclear clock. Thirolf and colleagues estimated energy by measuring the electrons emitted when the nucleus jumps between the two levels, as reported in Nature in 2019. And in a 2020 article in Physical Review Letters, physicist Andreas Fleischmann and colleagues measured other energy jumps what the thorium core can do. , subtracting them to deduce the energy of the nuclear clock jump.
The teams agree that the jump has just over 8 energy electrons. That energy corresponds to ultraviolet light in a range for which it is possible to start the jump with a laser, but at the edge of the capabilities of scientists.
Now that physicists know the size of the energy jump, they intend to shoot it with lasers. At the meeting, physicist Chuankun Zhang of the JILA research institute in Boulder, Colorado, reported efforts to use a frequency comb (SN: 10/5/18) – a method to create an array of discrete laser light frequencies – for start the jump and measure your energy even better. “If it’s a success, we can directly build a nuclear-based optical clock,” he said at the meeting. Thirolf’s team is also working with frequency combs, with the goal of making a nuclear clock work in the next five years.
Meanwhile, Pálffy is looking for what is called an "electronic bridge." Instead of using a laser to directly initiate a jump of energy through the nucleus, the laser first excited the electrons, which would then transfer energy to the nucleus, Palffy reported at the meeting.
Nuclear clocks could allow researchers to develop new evidence to determine whether the fundamental constants of nature vary over time. For example, some studies have suggested that the fine-structure constant, a number that establishes the strength of electromagnetic interactions, could change (SN: 11/2/16). “This nuclear clock is a perfect system for looking for variations of fundamental constants,” Victor Flambaum of the University of New South Wales in Sydney said at the meeting. The devices could also prove a foundation of Einstein's general theory of relativity called the equivalence principle (SN: 12/4/17). Or they could look for dark matter, hidden and undetected particles that physicists believe represent most of the matter in the universe, which could adjust 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 solve the dilemma of how scientists could build a nuclear clock, he says. It was "clear from the first second that this is an issue that should be worked on."