Researchers from Vienna University of Technology, under the direction of Professor Thorsten Schumm and a team from the National Metrology Institute Braunschweig (PTB), have successfully manipulated atomic nuclei with lasers for the first time. A key to the success was using a special crystal containing many thorium atoms. This research was published in the journal Physical Review Letters.
A laser beam changes the state of thorium nuclei, which are embedded in a crystal. Image Credit: Oliver Diekmann, Vienna University of Technology
Across the globe, scientists have dedicated years to the quest for a particular state of thorium atomic nuclei, anticipating its potential for groundbreaking technological advancements.
For instance, it could be used to construct a nuclear clock that can measure time more accurately than the most advanced atomic clocks on the market. Additionally, it might be applied to resolve entirely new basic questions in physics, such as whether or not the constants of nature are genuinely constant across space and time.
This hope has now materialized: the long-expected thorium transition has been discovered, and its precise energy is now known. It has been possible to precisely track the return of an atomic nucleus to its original state after transferring it into a higher energy state using a laser for the first time.
This allows for merging nuclear physics and classical quantum physics, two branches of physics that were not previously closely related. This achievement required, among other things, the creation of unique crystals containing thorium.
Switching Quantum States
Nowadays, manipulating atoms or molecules with lasers is common practice. If the laser's wavelength is precisely selected, atoms or molecules can be changed from one state to another. This method allows for the extreme precision of measuring atomic and molecular energies.
This is the foundation of many precision measurement techniques, including chemical analysis techniques and modern atomic clocks. Quantum computers also frequently use lasers to store data in atoms or molecules.
However, for a very long time, it did not seem like these methods could be used with atomic nuclei.
Atomic nuclei can also switch between different quantum states. However, it usually takes much more energy to change an atomic nucleus from one state to another – at least a thousand times the energy of electrons in an atom or a molecule, this is why normally atomic nuclei cannot be manipulated with lasers. The energy of the photons is simply not enough.
Thorsten Schumm, Professor, Vienna University of Technology
Atomic nuclei are much smaller than atoms and molecules and, hence, far less sensitive to outside disturbances like electromagnetic fields, so they are the ideal quantum objects for precise measurements. Therefore, they would theoretically enable measurements with never-before-seen accuracy.
The Needle in the Haystack
There have been rumors since the 1970s that thorium-229, a unique atomic nucleus, might be able to be altered with a laser in contrast to other nuclei. This nucleus has two extremely close energy states. In theory, changing the state of the atomic nucleus should only require the use of a laser.
However, there was little concrete proof of this transition's existence for a considerable time.
“The problem is that you have to know the energy of the transition extremely precisely to be able to induce the transition with a laser beam, knowing the energy of this transition to within one electron volt is of little use if you have to hit the right energy with a precision of one-millionth of an electron volt to detect the transition.”
Thorsten Schumm, Professor, Vienna University of Technology
The Thorium Crystal Trick
Some research teams have attempted to study thorium nuclei using electromagnetic traps one at a time. However, Thorsten Schumm and his group opted for an entirely different approach.
We developed crystals in which large numbers of thorium atoms are incorporated, although this is technically quite complex, it has the advantage that we can not only study individual thorium nuclei in this way but can hit approximately ten to the power of seventeen thorium nuclei simultaneously with the laser – about a million times more than there are stars in our galaxy.
Fabian Schaden, Professor, Vienna University of Technology
Schaden developed the crystals in Vienna and measured them together with the PTB team.
The effect is amplified, the necessary measurement time is shortened, and the likelihood of finding the energy transition is increased due to the large number of thorium nuclei.
Finally, on November 21, 2023, the team succeeded: The thorium nuclei produced a distinct signal for the first time, and the precise energy of the thorium transition was reached. In actuality, the laser beam had changed its state. The outcome has been released now that the data has been thoroughly inspected and assessed.
Since 2009, Schumm has devoted all of his research efforts to the hunt for the thorium transition. In recent years, his group and other competing teams worldwide have repeatedly achieved significant partial successes.
Schumm said, “[W]e are delighted that we are now the ones who can present the crucial breakthrough: The first targeted laser excitation of an atomic nucleus.”
The Dream of the Atomic Nucleus Clock
The discovery of how to excite the thorium state has opened up new and exciting avenues for research, including using this technology for precise measurements.
Thorsten Schumm said, “From the very beginning, building an atomic clock was an important long-term goal, similar to how a pendulum clock uses the swinging of the pendulum as a timer, the oscillation of the light that excites the thorium transition could be used as a timer for a new type of clock that would be significantly more accurate than the best atomic clocks available today.”
However, more than just time can now be measured with such high precision. For instance, it might be possible to analyze the Earth's gravitational field so precisely that it could reveal information about mineral resources or seismic activity.
The measurement technique may also be utilized to solve some of the most important physics mysteries, such as whether natural constants exist or if it possible to track minute changes over time.
Thorsten Schumm concluded, “Our measuring method is just the beginning, we cannot yet predict what results we will achieve with it. It will certainly be very exciting.”
Journal Reference:
Tiedau, J., et al. (2024) Laser Excitation of the Th-229 Nucleus. Physical Review Letters. doi.org/10.1103/PhysRevLett.132.182501
Source: https://www.tuwien.at/en/tu-wien