Prospects for a Nuclear Optical Frequency Standard Based on Thorium-229

Abstract

The 7.6-eV-isomer of Thorium-229 offers the opportunity to perform high resolution laser spectroscopy of a nuclear transition. We give a brief review of the investigations of this isomer. The nuclear resonance connecting ground state and isomer may be used as the reference of an optical clock of very high accuracy using trapped and laser-cooled thorium ions, or in a compact solid-state optical frequency standard of high stability.

Direct detection of the 229Th nuclear clock transition

“…Besides the low excitation energy, a radiative isomeric halflife in the range of minutes to hours has been predicted [13][14][15], resulting in a relative linewidth as low as ∆E/E ≈ 10 −20 . These unique features render this transition an ideal candidate for a nuclear clock [12], which may outperform existing atomic-clock technology due to potentially improved compactness and expectedly higher resilience against external influences [16]. Two ways to establish a nuclear clock are currently being investigated; one based on 229 Th 3+ stored in a Paul trap [17][18][19], the other one based on 229 Th embedded in a crystal-lattice environment [12,[20][21][22][23].…”

Nuclear clocks based on resonant excitation of γ-transitions

“…This was later discussed in detail in Refs. [30][31][32]. The idea of probing the 229 Th nuclear resonance in a solid receives increased attention, because this is regarded as a relatively simple way to prepare a bigger ensemble of nuclei for optical spectroscopy, even if there will be some line broadening and shifts.…”

Performance of a229Thorium solid-state nuclear clock

“…Other optimized parameters corresponding to R = 10γ are t 3.88/γ 5 × 10 3 s, t R 0.026 and δ m 0.99 . A few words on the validity of approximation (9) and the optimal value of the temperature: taking R = 10γ and using (15), we easily estimate the 'reasonable' value of V eg as V eg 2π · 0.53 Hz. The mixing rate gg should be large in comparison with this value.…”

“…Our estimation (see section 2.2) gives gg 2π · (5-100) Hz for room temperature, and gg ∝ T 2 ; then we can take gg [Hz] (5 × 10 −5 -10 −3 ) · T 2 [K ]. Condition (9) gives T [K ] > 0.53 5·10 −5 −10 −3 or T > 20-100 K. If (9) is not fulfilled, in a real multilevel system a significant fraction of nuclei will be pumped into the dark state, and growth of the excitation rate R will reduce the performance. On the other hand, even at liquid nitrogen temperatures the contribution of the temperaturedependent second-order Doppler broadening term to the total decoherence rate becomes comparable to the temperature-independent magnetic dipole interaction term, and increasing the temperature is undesirable.…”

“…Two main experimental approaches to a thorium clock, first proposed in [8], are under realization now: the use of trapped thorium ions [9][10][11] and 'solid-state nuclear spectroscopy' [12,13].…”