For the first time, scientists used an atomic nucleus as a clock.
The world’s most precise watches are made from atoms, specifically their electrons. But clocks based on atomic nuclei – protons and neutrons – could possibly surpass themwhile testing the fundamental laws of physics in new ways. Today, the decades-old dream of a nuclear clock was finally realizedreport two independent teams of researchers.
The technology is still in its infancy, but the physics behind it is so different from that of atomic clocks that it is already innovatedreport the researchers in an article submitted June 3 to arXiv.org. “In certain types of measurements we already outperform all atomic clocks,” says physicist Thorsten Schumm from TU Wien in Vienna.
Schumm and his colleagues used the clock to search for evidence of dark matter, the invisible, massive substance that is believed to permeate the universe. No signs of any suspicious substances materialized, but the clock’s estimated sensitivity to certain types of dark matter rivaled or exceeded that of atomic clocks.
“This is an exceptional result,” says theoretical physicist Victor Flambaum of the University of New South Wales in Sydney, who was not involved in the research. And this achievement should encourage further progress: “This is only the first step. [The] the race to build ultra-precise nuclear clocks has just begun.
The potential for nuclear clocks to impact dark matter and other exotic physics scenarios explains why, in this branch of physics, “nuclear clocks have become one of the most actively researched frontiers,” says Shiqian Ding of Tsinghua University in Beijing. In a study submitted June 7 to arXiv.org, Ding and his colleagues describe their nuclear clock, based on similar technology to that of Schumm and his colleagues. (Neither article has been peer-reviewed.)
At their heart, both clocks are made of calcium fluoride crystals impregnated with thorium, which are probed with a laser. Both clocks showed similar performance. Ding and his colleagues used a much more powerful laser, but Schumm and his colleagues had more thorium in their crystal.
And thorium is the key: in the entire periodic table, there is only one type of atomic nucleus that can be used to make a clock: thorium-229. This is because the core must integrate well with the other major player, the laser.
In atomic and nuclear clocks, the undulating electromagnetic waves of light from a laser act like the swinging pendulum of a grandfather clock. If this laser light were not anchored to something stable, its frequency would drift over time, as if the ticking of the grandfather clock were slowing down or speeding up. But in an atomic or nuclear clock, the laser frequency is locked to a jump between the energy levels of a particular atom or nucleus. For an atomic clock, it’s the electrons that make the jump, and for a nuclear clock, it’s the nucleus. Thorium-229 is special because it is the only atomic nucleus with an energy jump the right size to be probed by a laser.
To lock a laser into a jump in energy level, the laser must be frequently readjusted, using the results of the measurements to determine how to increase its frequency. In the new work, both teams managed to implement this feedback loop, a step that was absent in earlier demonstrations that laid the foundations for a nuclear clock.
“It was the last missing step before we could talk about a real clock,” says physicist Lars von der Wense, who was not involved in the research. Clocks did not yet surpass the best atomic clocks in timekeeping ability. But the technology is expected to advance quickly, with improvements in lasers and crystals on the horizon, says von der Wense, of the Johannes Gutenberg University of Mainz in Germany.
The arrival of nuclear clocks was eagerly awaited. Compared to atomic clocks, nuclear clocks are less sensitive to stray electromagnetic fields and can be made from solid materials, while atomic clocks require atoms to be suspended in a bulky vacuum chamber. This suggests possibilities for making more portable and robust ultra-precise clocks.
In addition, atomic nuclei are subject to forces different from those of electrons, which opens up new avenues of study. Protons and neutrons are held together by the strong nuclear forcewhile electrons are mainly subject to electromagnetic forces. Numbers called fundamental constants determine the relative strength of these forces, and comparisons between an atomic clock and a nuclear clock could be used to look for variations in these numbers over time. These variations could be due to ultralight dark matter – the subject of Schumm and his colleagues’ research.
It took a long time for scientists to come up with the idea of a thorium nuclear clock in 2003. But “I have always been optimistic that this project will succeed,” says physicist Ekkehard Peik of the National Metrology Institute in Braunschweig, Germany. Peik is one of the scientists who proposed the idea and co-authored with Schumm on the new paper.
After initially slow progress, researchers have made rapid progress in recent years. Today, Peik says, “I am confident that this momentum will continue and that a lot of interesting research… is just beginning.” »
