Two-Photon Electron-Nuclear Resonance as a Bridge to Nuclear-Optical Clocks

F. F. Karpeshin, L. F. Vitushkin

Transactions of IAA RAS, issue 74, 37–41 (2025)

DOI: 10.32876/ApplAstron.74.37-41

Keywords: nuclear optical frequency standard, thorium-229 isomer, optical pumping

Abstract

The best samples of atomic clocks demonstrate a relative error of several 10⁻¹⁸ units. However, solving complex fundamental and applied problems requires reducing this error by an additional order of magnitude. Achieving further error reduction could help address the long-standing issue of potential drift in fundamental constants, as well as shed light on the mysteries of dark matter and dark energy. Consequently, significant attention in current scientific research is focused on developing nuclear-optical clocks and, accordingly, next-generation frequency standards. In 2024, a major milestone was reached with the successful excitation of the 8.355740(3) eV nuclear isomer 229mTh using laser techniques, thanks to collaborative efforts by physicists from PTB, LMU Munich, JILA, UCLA, and others. The candidacy of this nuclide for creating the desired standard has been confirmed. The energy of its excited state 3/2⁺[631], is only 8.355740(3) eV higher than the ground state, 5/2⁺[633]. However, the currently used technologies are extremely low in resource efficiency. Only about 1/10¹³ of the consumed power contributes to exciting the isomer; the rest is lost in the resonator, amplifiers, unused harmonics, and modes of the frequency comb, among other sources. There is ongoing discussion about further refining the isomer’s energy to reduce the error via resonant optical pumping, utilizing the resonant properties of the electron shell. In principle, this could yield a gain of up to 13 orders of magnitude — far greater than what current methods achieve. This paper emphasizes accounting for the resonance width. Proper consideration can either enable increasing the scanning step size — thus reducing the total scanning time — or simply enhance the cross section. The proposed twophoton method leverages a significant broadening of the isomer line caused by mixing with an electron transition. This approach does not suffer from the typical reduction in cross section associated with broadening — such as that caused by internal conversion or intentional broadening of the pump laser’s spectral line. The scheme under consideration is two orders of magnitude more efficient than direct pumping and is applicable to both ionized and neutral thorium atoms. Its implementation involves exciting both the nucleus and the electron shell in the final state.

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