For the first time in history, researchers have used laser light to push the nucleus of a thorium atom to a higher energy level. This scientific feat opens the door to a whole new generation of clocks of unparalleled precision that could help us probe the universe’s most fundamental forces.
How Do Atomic Clocks Work?
Atomic clocks, which are currently the most precise instruments we possess, rely on a captivating principle of quantum physics. They use atoms to measure time with extreme precision.
Picture an atom as a tiny solar system in miniature, with a nucleus at its heart and electrons orbiting around it. These electrons can occupy distinct energy levels, much like the floors of a building. When an electron gains a certain amount of energy, it can “jump” from one level to another. This is what we call an energy transition.
Now, imagine a laser, a focused beam of light. If we tune the laser’s frequency to match precisely the energy required to trigger this energy jump for a given electron, something quite extraordinary occurs. The electron absorbs the laser energy and leaps to a higher energy level. Yet this state is unstable, so the electron quickly returns to its original state by emitting a photon or a small packet of light.
This phenomenon is particularly interesting for a clock: the frequency at which these photons are emitted is extremely precise and constant. It directly depends on the energy difference between the atom’s electronic levels. By measuring this frequency, we can thus obtain a highly precise measure of time.
To put it simply, it’s a bit like each tick of the clock being the result of an electron performing a small energy jump inside the atom.
The Promise of Nuclear Clocks
However, these atomic clocks have their limits. The electrons they rely on reside outside the nucleus, making them vulnerable to external disturbances such as stray magnetic fields or other environmental effects. These perturbations can subtly alter the electrons’ energy levels and, consequently, affect the clock’s running time.
This is where the nuclear clock comes into play. Unlike atomic clocks, it would exploit the energy transitions of the nuclei inside the atoms, thereby shielding them from external interferences.
Yet until recently, the challenge was that the gaps between nuclear energy levels are far larger than those of the electrons, making it difficult to use lasers to drive these transitions. This brings us back to these latest efforts. Physicists have recently used laser light to push the nucleus of a thorium atom to the desired energy level.
New Breakthrough
In the 1970s, scientists made a fascinating discovery regarding a particular thorium isotope, thorium-229. They observed that this isotope possessed an energy level that appeared to lie within the range accessible to laser light. In other words, laser light held the potential to excite thorium-229 to a higher energy level.
Nevertheless, pinning down this specific energy gap proved to be a major challenge. To achieve it, researchers had to excite thorium-229 to an energy level far above the two levels that genuinely interested them. Then they measured the subtle differences in the energy of the light emitted when thorium-229 fell back to the higher of the two levels of interest.
After decades of effort and research, the scientists finally reached their goal. They succeeded in directly observing a thorium-229 atom making this jump between energy levels, thereby demonstrating a precise energy change of 8.35574 electron-volts.
To observe this thorium-229 atom directly, the researchers employed a sophisticated method that involves trapping the atoms inside calcium fluoride crystals. By confining them in a controlled environment, they were able to create the ideal conditions to observe the energy-level jump.
Prior to this breakthrough, researchers were constrained by the technology available to trap and manipulate thorium-229 atoms with precision. Advances in techniques and the manipulation of atoms were therefore essential to enable this direct observation.
This advancement represents a leap forward in our ability to probe the mysteries of the universe with unprecedented precision. With this command of the thorium-229 energy transitions, we are approaching an era where nuclear clocks will measure time with unrivaled accuracy. Beyond merely telling time, these revolutionary instruments could also illuminate fundamental questions about the very nature of the universe, helping us explore phenomena such as dark energy and dark matter with a new level of scientific finesse.
The details of the study are published in the journal Physical Review Letters.
