New state of thorium opens possibilities for nuclear clocks

Why do we have atomic clocks but not nuclear clocks? After all, the nucleus is usually surrounded by many electrons, so in principle it is less susceptible to external noise (in the form of light) It should be. For atoms with higher atomic numbers, the nucleus contains more particles than the element has electrons. It holds almost the entire mass of the atom, even though it occupies only about 1/100,000th of the space of the atom. The first atomic clock was invented in 1949, but no nuclear clock has yet been put into practical use.

The reason is simple: it takes much more energy to excite an atomic nucleus to a higher energy state than an atom. Atomic clocks typically excite cesium atoms with photons of energy of 4 x 10-5 electron volts. The most promising nucleus for a nuclear clock is currently thought to be thorium-229, whose nuclear excited state requires a photon of about 8 eV, more than 200,000 times higher. They are microwaves and ultraviolet rays. Additionally, the interaction between light and atomic nuclei can be weak.

Now, Chinese scientists have discovered that the interaction between light and atomic nuclei is much stronger and more efficient. More than 10% of the highly ionized thorium-229 nuclei can be excited with a single laser pulse. Thorium normally has 90 electrons (atomic number) around its nucleus, but the research group removed all but one of these electrons to remove one from every proton in the nucleus. It produced thorium, which is like hydrogen and has a charge of +89 minus an electron. Their research is published in Physical Review Letters.

Existing atomic clocks operate using the resonant frequencies of atoms. A group of atoms in one of two energy states is used to pump as many atoms as possible to a higher energy level by shining light from a laser of the appropriate energy on the lower energy group.

The more precise the frequency of the radiation, the more atoms will jump to higher states. Different types of states are separated, and high-energy atoms decay into lower-energy states that emit light at the same natural frequency. For a cesium-133 atom, this frequency is exactly 9,192,631,770 Hertz. The incident light raises the atoms to a higher energy level again, after which they collapse and so on.

If you tune the frequency of the incident light enough, the largest low-state atoms will resonate and transition back to the high state. and so on. In this way, atoms can be used as oscillators, the basis of all clocks.

Lead author Hanxu Zhang, from the Chinese Society of Engineering Physics in Beijing, and his colleagues have shown that currently available high-intensity lasers and the hydrogen-like thorium atom 229Th89+ may be able to achieve the resonance needed for nuclear analogs of atomic clocks. It showed that there is. This nuclear isomer (metastable nuclear state) can be excited with a probability of 10% using a powerful laser pulse of 1021 watts/cm2.

As these highly ionic thorium atoms return to a lower energy state, they emit light at multiple wavelengths that are harmonics (integer multiples of frequency) of the isomer transition frequency within about 0.01 seconds. (The transition time for bare thorium nucleus 229Th90+ is approximately 1,000 seconds.)

This provides new opportunities for nuclear-based coherent light emission, similar to lasers, where the frequency is the same but there is a constant phase difference between light from different transitions.

The transition probability of 10% is a highly nonlinear change from the typical energy transition of isomers with nuclear hyperfine mixing induced by the magnetic field of a single 1s electron on thorium-89+. (The magnetic dipole moment of the electron is about 1,000 times larger than the magnetic dipole moment of the nuclear state.)

Its powerful magnetic field finely modulates (“splits”) the nuclear energy level of the electron-free isotope Th90+, significantly shortening the lifetime of the upper state by a factor of about 100,000, according to the numbers above. This resonance could pave the way for nuclear optical clocks.

The researchers found that by increasing the laser energy intensity by four orders of magnitude, the probability of excitation (to the isomeric state thorium-89+) by a single laser pulse changed from a linear variation with energy to a highly nonlinear variation that increased by 14 orders of magnitude. I discovered that. (See diagram above). After that, there is no further increase in energy intensity.

In this way, the probability of nuclear energy transition increases from approximately 10-15 to 10-1 (10%). Together with the resonant frequency, this significant increase paves the way for the realization of nuclear clocks.

“These results open new frontiers in light-matter interactions and provide exciting perspectives on the manipulation of atomic nuclei by light,” the group said. “The discovery of a new mechanism for nuclear coherent light emission It has broader implications across diverse research fields.” ”

They point out that their results can be easily implemented with experimental equipment that exists today, “underscoring its practical relevance and immediate applicability.”

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