The Nuclear-Powered Clocks of the Future

The Nuclear-Powered Clocks of the Future

What is the most accurate way to measure the passage of time? While the old "one Mississippi, two Mississippi" trick may suffice in a pinch, there are many scientific and technological applications that require much more precise timekeeping. Today, the best atomic clocks can tick so precisely that it would take longer than the age of the universe for them to be off by one second. However, some scientists are searching for an even more accurate clock, and they have proposed a literal nuclear option: a clock the size not just of an atom, but of an atom's nucleus.

To create any clock, one needs something that goes back and forth repeatedly at a fixed rate, which is called an oscillator, and it has a frequency, which is how many times it oscillates over a specified length of time. The most accurate and reliable oscillators are light waves, which are made of electric and magnetic fields that wobble at fixed frequencies. But how do light waves and atoms come together to make an atomic clock? The answer lies in quantum mechanics.

Atoms are made of a nucleus surrounded by a swarm of electrons, and those electrons can only exist where they have specific fixed energies. These energy levels can be thought of as rungs on a subatomic-sized ladder, and if an electron wants to climb up or down that ladder, it has to either gain or lose the exact amount of energy that will get it to another rung. Electrons can get that energy by either absorbing or emitting light with the appropriate frequency, because according to quantum mechanics, the energy and frequency of light are directly related. To make an atomic clock, scientists take a bunch of identical atoms, like cesium-133, and hit them with a laser that has a frequency and therefore an energy that is as close as possible to the energy needed to bump just one of each atom's electrons up to another rung. Any electrons that do get a boost will eventually shed their excess energy and drop to their original rung, and that means that they will emit light on their way down again. That light oscillates, or ticks, at a precise frequency, and scientists count those incredibly consistent ticks to mark the passage of time.

With decades of laser science research behind them, atomic clocks are simple enough that we can shoot them into space and they'll still be accurate down to the nanosecond. But we can do even better, because atomic energy levels aren't the only game in town. An atom's nucleus has its own even smaller subatomic ladder, and just like electrons, a nucleus can jump from one of its proverbial rungs to another if it absorbs or releases just the right amount of energy. When keeping track of time, a nuclear clock can offer some advantages over traditional atomic ones. While an electron's ladder has rungs at very specific energy levels, those levels aren't always constant. The position of the rungs can shift by tiny amounts if there's a slight shift in some external electric or magnetic field, and if your clock is the size of a single atom, those tiny shifts are a big deal and can throw off your timekeeping ability. But nuclear energy levels are less affected by changes going on around them because the protons and neutrons inside an atom's nucleus are so tightly bound together. These super-strong bonds can help a nuclear clock tick more steadily than an atomic clock. However, there's a catch: the gaps between the energy levels, the distance between the rungs on the ladder, are on a completely different scale. Typically, we're looking at energies that are millions of times greater than what electrons are dealing with. And remember, the amount of energy needed to jump between rungs relates to the frequency of the light we need to shine on them. For the average nucleus, you'd need to blast it not with the microwaves of traditional cesium atomic clocks or the optical lasers that we use with more state-of-the-art clocks that use other elements. No, you would need a laser that shoots gamma rays, the most energetic and highest frequency light out there. And that's just not feasible with current technology.

As luck would have it, there is one nuclear energy gap we know of that could work. It's found in the radioactive element thorium-229. The problem is that while scientists know the gap corresponds to some kind of ultraviolet light, no one's been able to pin down the exact energy. Without knowing that, you can't excite any thorium nuclei inside your fancy new clock. But a big breakthrough came in 2023 when a team at CERN used a somewhat conventional approach for their experiments. Instead of taking thorium nuclei and trying on a bunch of different laser frequencies to see which one excites them