The new results suggest the possibility of combining classical quantum physics and nuclear physics

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The laser beam hits the thorium nuclei embedded in the crystal. Credit: Tu Win

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The laser beam hits the thorium nuclei embedded in the crystal. Credit: Tu Win

Physicists have been hoping for this moment for a long time: For many years, scientists around the world have been searching for a very specific case of thorium atomic nuclei that promises revolutionary technological applications. They could be used, for example, to build a nuclear clock that can measure time more accurately than the best atomic clocks available today. It can also be used to answer completely new fundamental questions in physics, for example, the question of whether the constants of nature are actually constant or whether they change in space and time.

Now that hope has become a reality: the long-awaited transformation of thorium has been discovered, and its energy is now precisely known. For the first time, it became possible to use lasers to move an atomic nucleus to a higher energy state and then precisely track its return to its original state.

This makes it possible to combine two areas of physics that previously had little to do with each other: classical quantum physics and nuclear physics. A prerequisite for achieving this success was the development of special crystals containing thorium.

Now a research team led by Professor Thorsten Schumm from TU Wien (Vienna). published This success is in collaboration with a team from the National Institute of Metrology in Braunschweig (PTB) in the magazine Physical review letters.

Switching quantum states

Manipulating atoms or molecules with a laser is common today: if the laser wavelength is chosen just right, atoms or molecules can be converted from one state to another. In this way, the energies of atoms or molecules can be measured very precisely. Many precise measurement techniques rely on this, such as current atomic clocks, as well as methods of chemical analysis. Lasers are also often used in quantum computers to store information in atoms or molecules.

However, for a long time it seemed impossible to apply these techniques to atomic nuclei.

“An atomic nucleus can also transition between different quantum states,” says Schumm. “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 the electrons in an atom or molecule.” “This is why atomic nuclei cannot be manipulated with a laser. The energy of photons is simply not enough.”

This is unfortunate because atomic nuclei are actually ideal quantum objects for precise measurements: they are much smaller than atoms and molecules, and therefore less susceptible to external disturbances, such as electromagnetic fields. In principle, it will allow measurements with unprecedented precision.

PTB researcher Johannes Tedau in the laser laboratory. Credit: BTB Braunschweig

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PTB researcher Johannes Tedau in the laser laboratory. Credit: BTB Braunschweig

The needle in the haystack

Since the 1970s, there has been speculation that there might be a special atomic nucleus, which, unlike other nuclei, could perhaps be manipulated by a laser, namely thorium-229. This nucleus has two energy states so closely adjacent that a laser would in principle be sufficient to change State of the atomic nucleus.

But for a long time, there was only indirect evidence of the existence of this shift. “The problem is that you have to know the energy of the transformation very precisely to be able to create the transformation with a laser beam,” says Schumm.

“Knowing the energy of this transition to within one electron volt would not be of much use if you had to access the correct energy with an accuracy of a millionth of an electron volt in order to detect the transition.” It's like looking for a needle in a haystack, or trying to find a small treasure chest buried on a kilometer-long island.

Thorium crystal trick

Some research groups have attempted to study thorium nuclei by holding them individually in place in electromagnetic traps. However, Shum and his team chose a completely different approach.

“We have developed crystals in which large numbers of thorium atoms are incorporated,” explains Fabian Schaden, who developed the crystals in Vienna and measured them in collaboration with the PTB team.

“Although this is technically very complex, it has the advantage that not only can we study individual thorium nuclei in this way, but we can also hit approximately 10 to the power of 17 thorium nuclei simultaneously with a laser – that is, about a million times.” More than the stars in our galaxy.

The large number of thorium nuclei amplifies the effect, shortening the measurement time required and increasing the probability of actually finding the energy transition.

Thorsten Schumm (Tu Wien, Vienna) holds one of his crystals. Credit: Photowell

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Thorsten Schumm (Tu Wien, Vienna) holds one of his crystals. Credit: Photowell

On November 21, 2023, the team finally succeeded: the correct energy for the thorium transition was reached exactly, and the thorium nuclei sent a clear signal for the first time. The laser beam has already changed the state. After careful examination and evaluation of the data, the result has now been published.

“For us, this is a dream come true,” Shum says. Since 2009, Schaum has focused his research entirely on researching thorium transport. His group as well as competing teams from around the world have repeatedly achieved important partial successes in recent years.

“Of course, we are delighted that we are now able to deliver the decisive breakthrough: the first targeted laser excitation of an atomic nucleus,” says Schumm.

Dream of the atomic nucleus clock

This marks the beginning of an exciting new era of research: now that the team knows how to excite the thorium state, the technique can be used to make precise measurements. “From the beginning, building an atomic clock was an important long-term goal,” says Schumm.

“Similar to the way a pendulum clock uses the swing of the pendulum as a timer, the oscillation of light that excites the transmission of thorium could be used as a timer for a new type of clock that would be far more accurate than the best atomic clocks available today.”

But it is not only time that can be measured in this way more precisely than before. For example, the Earth's gravitational field can be analyzed so accurately that it can provide indications of mineral resources or earthquakes. The measurement method can also be used to access fundamental mysteries of physics: Are the constants of nature really constant? Or can small changes over time be measured?

“Our measurement method is just the beginning,” says Shum. “We can't yet predict what results we will achieve with it. It will certainly be very exciting.”

more information:
J. Tedau et al., Laser Excitation of Th-229 Nuclei, Physical review letters (2024). doi: 10.1103/PhysRevLett.132.182501

Magazine information:
Physical review letters

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