The most accurate clocks humans have ever built all run on the same basic trick. You take an atom, hit it with laser light tuned to the exact color that makes one of its electrons jump between two energy levels, and you count the beats.<\/p>\n
Do that precisely enough and you get a machine so steady the best ones would lose about a second every 30 billion years. That is the electron doing the timekeeping.<\/p>\n
Two teams of physicists just built a clock that ignores the electrons and reads time off the nucleus instead.<\/p>\n
The nucleus of one isotope, thorium-229, has its own tiny, laser-triggerable jump buried deep inside the atom. Getting a laser to hit it, and then getting the whole thing to hold steady on its own, has taken the better part of fifty years.<\/p>\n
In early June, a group in Vienna and a group at Tsinghua University in Beijing crossed that line within days of each other, and did it independently. Physicists are calling the result the first working nuclear clock, and the honest version of that claim is more interesting than the headline.<\/p>\n
An atomic clock and a nuclear clock are chasing the same thing: a rock-solid oscillation to count. In a normal optical atomic clock, that oscillation is the frequency of light an electron soaks up when it hops to a higher energy level. Lock a laser to that frequency, count the waves, and you have a ruler for time.<\/p>\n
A nuclear clock swaps the electron for the nucleus. Thorium-229 has an excited state sitting at an unusually low energy, low enough that light can nudge the nucleus into it.<\/p>\n
The color you need is a sliver of vacuum ultraviolet at roughly 148 nanometers, which you cannot get straight out of an ordinary laser. Both teams packed the thorium into a small calcium fluoride crystal, doped so densely with the isotope that a beam actually has something to hit, and left the crystal sitting at room temperature.<\/p>\n
The part that turned a physics demo into an actual clock was a feedback loop. Rather than finding the nuclear transition once and walking away, the setup constantly checks whether the laser has drifted off the nucleus and nudges it back, so the nucleus itself is steering the laser.<\/p>\n
Lars von der Wense, a physicist at Johannes Gutenberg University Mainz, told Science News it was “the final missing step before calling it an actual clock.” The Vienna group compared their stabilized signal against an ytterbium-ion clock to measure how well it held.<\/p>\n
The two papers went up on the preprint server arXiv on June 3 and June 7, from the Vienna team led by Thorsten Schumm<\/a> and the Tsinghua team led by Beichen Huang<\/a>. Neither has cleared peer review yet.<\/p>\n So why go to all this trouble to move the tick a few femtometers inward? Because the nucleus is a fantastically well-protected place to keep time.<\/p>\n An atom’s nucleus is orders of magnitude smaller than the atom around it, and it sits wrapped in its own cloud of electrons that screens it from the outside world. Stray electric and magnetic fields, small temperature wobbles, the stuff that jostles an electron-based clock and forces heroic engineering to cancel out, barely reach the nucleus at all.<\/p>\n The transition also lives inside a solid crystal instead of floating as a single trapped ion. That is a big part of why physicists think these clocks could eventually be tougher and more portable than the room-sized instruments that hold the current records.<\/p>\n That reliability is the same reason a well-understood nuclear property makes such a dependable stopwatch in general. We have written about a coin-sized battery that runs for a century on the steady decay of nickel-63<\/a>, where the entire pitch rests on a half-life being one of the best-characterized numbers in physics.<\/p>\n The clock is a cousin of that idea. Except instead of counting how fast atoms fall apart, it counts a single clean jump inside a nucleus that is not falling apart at all.<\/p>\n Getting here was slow. The thorium state was first predicted back in the 1970s, and for decades nobody could pin down its energy well enough to aim a laser at it. That only cracked open in 2024, when physicists excited the transition with a laser for the first time and measured its wavelength to within a hair. The clock is what you get two years later, once you add the loop that keeps it honest.<\/p>\n The coverage keeps glossing over one detail. The nuclear clocks that just switched on are not more precise than the best atomic clocks. Not yet.<\/p>\n The Vienna group reports their clock’s stability approaches about a part in a quadrillion over a full day of running. That is genuinely good. But the best optical atomic clocks are roughly a thousand times steadier, the ones losing a second every 30 billion years, so nothing has been dethroned here.<\/p>\n What the nuclear clock has is headroom. The same paper projects that future solid-state versions could improve by several orders of magnitude as the crystals, lasers and readout get better.<\/p>\n The current number is a starting line, not a ceiling, and the whole reason the field is buzzing is that the nucleus should in principle let these clocks blow past where atomic clocks top out. Gilad Perez, a theoretical physicist at the Weizmann Institute of Science, told Nature the technology has crossed from a promising idea to “a functioning precision instrument.”<\/p>\n Nobody is building these to make the clock on your phone nicer. The interesting uses fall into two buckets.<\/p>\n The first is navigation and sensing. Every GPS fix you have ever gotten leans on atomic clocks in orbit, and steadier, tougher, more portable clocks feed straight into better positioning, better timing for networks, and gravity sensors that can read tiny differences in the pull of the Earth.<\/p>\n Timing is position, when you get down to it. Light covers about a foot in a nanosecond, so an error in the clock is an error in where the system thinks it is standing. A clock sealed inside a crystal is a far easier thing to bolt onto a ship, a plane or a truck than a lab full of trapped ions.<\/p>\n The second use is stranger, and it is the one physicists actually care about. The thorium transition is unusually sensitive to the forces at work inside the nucleus, which makes a good nuclear clock double as a detector for physics our current theories do not explain.<\/p>\n The Vienna team already pointed theirs at ultralight dark matter, hunting for tiny, telltale wobbles in the nuclear frequency over stretches from 20 seconds to a day. Those first results already compete with atomic clocks on part of that search and push further on how dark matter might couple to the strong force.<\/p>\n That is the real prize. A clock this sensitive is a way to test whether the constants of nature actually stay constant, and to go looking for a fifth force sitting outside the physics we have now. It just happens to also tell time.<\/p>\n None of this is a finished product. Both results are preprints, so they still have to clear peer review, and turning a lab bench into a rugged, shrinkable instrument is its own multi-year slog. The current clocks also still lean on supporting hardware, including that ytterbium-ion reference the Vienna team checked against.<\/p>\n It is not a solo win either. A near-simultaneous finish out of Vienna and Beijing is the mark of a field that went from a sleepy corner of physics to a genuine race, with more groups working the same crystal-and-laser problem from different angles.<\/p>\n For what it is worth, this is the third time lately we have watched someone turn one quirk of one isotope into a working device, after the UK’s carbon-14 diamond battery that trickles power on a 5,700-year clock<\/a>. The batteries run on how predictably a nucleus falls apart. The clock runs on a single clean jump inside a nucleus that stays put. Same raw material, opposite trick: one counts a nucleus coming apart, the other counts a nucleus holding together.<\/p>\n","protected":false},"excerpt":{"rendered":" The most accurate clocks humans have ever built all run on the same basic trick. You take an atom, hit … <\/p>\nA buried nucleus is a lot harder to rattle<\/h2>\n
It is not the most accurate clock on Earth yet<\/h2>\n
The payoff is not a better wristwatch<\/h2>\n
Still a preprint, and still a race<\/h2>\n