"EXACT CLOCK"

The insulating sphere around the thorium atom will help create a nuclear clock.

A physicist from the Moscow State University considered how the imposition of boundary electromagnetic conditions on the transition between the ground state and the excited state of the 229Th atom nucleus affects the surrounding of the atom with an insulator, placing it in a thin dielectric film or metal cavity. It turned out that in some cases the transition can be slowed down more than tenfold. The discovery may find application in the development of nuclear clocks, the accuracy of which will exceed the accuracy of atomic clocks (there are no estimates of the increase in accuracy in the article). The article is published in Physical Review Letters, it is briefly reported by Physics.

When scientists need to accurately measure time, they turn to atomic clocks. The construction of this watch is based on electron transitions between adjacent electron shells of the atom - the frequency of such oscillations is constant, and the relative error in its measurement is no more than εν ~ 10-14. In other words, the atomic clock accumulates an error of one second in a few million years. The atomic clock is used for precise position determination (GPS and GLONASS systems), installed at mobile base stations. In addition, the modern definition of a second also relies on atomic clocks: in the international system of SI units, the second is the time interval during which 9192631770 oscillations occur, corresponding to the resonant frequency of the energy transition between the levels of the hyperfine structure of the ground state of the cesium-133 (133Cs) atom.

However, for accurate measurement of time intervals, it is not necessary to use only a classical atomic clock. Theoretically, even greater accuracy can be achieved by observing transitions between the energy levels of the atomic nucleus, rather than electrons rotating around it. To do this, you need to learn how to control the probability of the process. Such control can be achieved by imposing on the atom boundary electromagnetic conditions - simply put, placing it in a "cage" and limiting its ability to radiate. For the first time this effect was predicted by Edward Parcell in 1946, by now it has been studied quite well theoretically and experimentally. Unfortunately, no one succeeded in using it to create nuclear precision watches. The obstacle here is the fact that the Parsell effect is rather difficult to see - the distance between the nuclear energy levels is much greater than the energy of optical atomic excitations and the electromagnetic radiation released as a result of the transition practically does not notice the boundary between the two media. Moreover, most of this energy goes not to radiation, but to internal conversion, which makes observations even more difficult.

Fortunately, there are exceptions to these patterns. In 1976, L. A. Kroger and C. W. Reich discovered a nucleus whose excited state had a relatively small energy. This state was the state of the thorium-229 (229Th) atom, which has a spin of 3/2 and a positive parity (spin 3/2 +). Recent measurements of the transition in such nuclei give an energy value of about E ~ 10 electron volts. In this paper, the physicist Yevgeny Tkalya theoretically examined the role played by the imposition of boundary electromagnetic conditions on 229Th atoms, and showed that in some cases they make it possible to slow down the transition (that is, to reduce its probability) by more than ten times.

In his work, the scientist considers four different types of boundary conditions: the surrounding of an atom with a dielectric (insulator) embedded in a vacuum or filling a cavity in a metal volume, placing the atom in a thin dielectric film attached to the semiconductor surface, and finally placing the atom in an empty metal cavity . In all cases with a dielectric, the physicist assumed that the energy gap of the conduction electrons was much greater than the energy of the photons that are emitted during the transition between the states of the nucleus, and no internal conversion occurred.

In early March, American physicists increased the accuracy of atomic clocks by almost one and a half times - the watches they designed "go off" by no more than a second for several hundred billion years. To achieve this accuracy, scientists simultaneously measured the state of not one but several thousand strontium atoms ordered by a laser into a three-dimensional lattice. However, it is always worth remembering that the existence of absolutely accurate clocks is forbidden by fundamental physical laws: the more accurately they measure time, the more free energy passes into heat, that is, the faster the entropy of the system increases.