Tiny Nuclear Clock In A Crystal Joins The Hunt For Dark Matter

Radioactive Crystal Clock Now Rivals Top Atomic Clocks in Dark Matter Hunt

A tiny radioactive crystal has become a clock sensitive enough to compete with some of the best atomic clocks on Earth in the search for dark matter, the invisible material that makes up most of the universe’s matter but has never been directly detected. The new thorium-229 nuclear clock, built by a team in Vienna, locks itself onto a faint flicker deep inside a thorium nucleus and corrects itself as it runs, a first for this kind of experiment. Results appear in the journal Nature.

Physicists have chased the idea of a nuclear clock since 2003, when Ekkehard Peik and Christian Tamm proposed that an oddly low-energy quirk inside the thorium nucleus could serve as a timekeeping signal. Because the flicker lives in the nucleus rather than the outer electrons that ordinary atomic clocks rely on, outside interference should disturb it far less, and the thorium can sit inside a solid crystal at room temperature.

What sets this work apart is the feedback loop, plus the payoff in the dark matter hunt. For certain possible interactions involving the strong nuclear force, the clock probed ten to a thousand times more sensitively than earlier atomic clock experiments. For interactions involving light, it matched the best atomic clocks.

Nuclear Clock Locks Onto Thorium Signal Without Outside Help

Top atomic clocks typically hold atoms or ions in vacuum chambers and cool them to extremely low temperatures. This clock skips that. Thorium atoms sit embedded in a clear calcium fluoride crystal only a few millimeters across. An ultraviolet laser shines through it. When the laser hits exactly the right frequency, the thorium nuclei absorb a sliver of the light, and a detector registers the dip.

Staying on that frequency is the hard part, since lasers drift. The system nudges the laser back and forth, checks which way it has strayed, and steers it back, much like a driver correcting course to stay in a lane. A correction can come as often as every 20 seconds, with each taking about a second to apply. Speed matters because a thorium nucleus holds its extra energy for roughly 10 minutes, and earlier methods that watched for a glow had to wait that long. The clock then ran for about a day without anyone stepping in, checked against a trusted atomic clock in Austria.

Nuclear Clock Finds No Dark Matter Signal but Narrows the Search

Some theories hold that dark matter consists of extremely light, undiscovered particles. Such particles would barely interact with ordinary matter, but they could make the strength of nature’s forces wobble slightly in a steady rhythm, which would shake the frequency of the thorium flicker too.

Thorium makes an unusually sensitive probe. Its nucleus has such low energy because two enormous forces, the electric repulsion among its protons and the strong nuclear force holding it together, nearly cancel out. A small change in either force throws off that balance, and theory predicts the thorium flicker reacts far more strongly than the electron-based signals in ordinary atomic clocks. That sensitivity lets the clock compete despite being less steady than the best atomic clocks.

Researchers combed through about 23 hours of data for rhythmic wobbles lasting 20 seconds to a full day, plus a slow drift. No wobble rose above the noise that computer simulations predicted from the equipment alone, and the drift was consistent with zero. That does not rule out the particles, but it caps how strongly they could interact with ordinary matter, assuming they make up most of the dark matter.

Day-to-Day Results Drift, and Crystal Flaws May Be to Blame

This is an early-stage device, and it shows. Within a single run, the clock behaved smoothly. From one day to the next, results shifted noticeably. The laser must be realigned before each run, so the beam crosses a slightly different part of the crystal every time.

To test that explanation, researchers measured four different spots. The thorium signal moved from spot to spot by as much as 13 times what measurement noise alone would explain, while returning to one spot brought back the same frequency. The authors suspect small strains from uneven crystal structure, a hypothesis not yet proven. More uniform crystals, or a beam confined to one reproducible spot, could help.

Stronger Lasers and New Crystals Could Close the Gap With Atomic Clocks

A stronger ultraviolet laser, crystals with thorium built into the material, and a sharper nuclear signal could bring a future thorium clock to the stability of today’s best atomic clocks, according to the paper’s projections. Sensitivity to slight shifts in the basic numbers that set nature’s rules could climb roughly ten-thousand-fold over this version.

A clock built around a crystal small enough for a lab bench is holding its own, in this search, against instruments that took decades to refine. Whether it catches a real signal or only keeps shrinking the places one could hide, it has earned a spot in the hunt.

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