With an energy of 8.355733554021(8) eV,[3][4][5] this corresponds to a frequency of 2020407384335±2 kHz,[6] or wavelength of 148.382182883 nm, in the vacuum ultraviolet region, making it accessible to laser excitation.

Their underlying principle of operation is based on the fact that the difference between two electron energy levels is independent of space and time.

[15] The achieved accuracies of these clocks vary around 10−18, corresponding to about 1 second of inaccuracy in 30 billion years, significantly longer than the age of the universe.

[16] In thorium cations, internal conversion is energetically prohibited, and 229mTh+ is forced to take the slower path, decaying radiatively with a half-life of around half an hour.

[4] Thus, in the typical case that the clock is designed to measure radiated photons, it is necessary to hold the thorium in an ionized state.

[17] In this case, the atoms are not 100% ionized, and a small amount of internal conversion is possible (reducing the half-life to approximately 10 minutes[4]), but the loss is tolerable.

[7] Such clocks are expected to achieve the highest time accuracy, as the ions are to a large extent isolated from their environment.

As the nucleus is largely unaffected by the atomic shell, it is also intriguing to embed many nuclei into a crystal lattice environment.

[19] It has also been proposed to irradiate a metallic 229Th surface and to probe the isomer's excitation in the internal conversion channel, which is known as the internal-conversion nuclear clock.

As early as 1996 it was proposed by Eugene V. Tkalya to use the nuclear excitation as a "highly stable source of light for metrology".

In their pioneering work, Peik and Tamm proposed to use individual laser-cooled 229Th3+ ions in a Paul trap to perform nuclear laser spectroscopy.

Instead of exciting an electronic shell state in order to obtain the highest insensitivity against external perturbing fields, the nuclear clock proposed by Campbell et al. uses a stretched pair of nuclear hyperfine states in the electronic ground-state configuration, which appears to be advantageous in terms of the achievable quality factor and an improved suppression of the quadratic Zeeman shift.

[38] This frequency of light is relatively easy to work with, so many direct detection experiments were attempted which had no hope of success because they were built of materials opaque to photons at the true, higher, energy.

[34] In 2019, the isomer's energy was measured via the detection of internal conversion electrons emitted in its direct ground-state decay to 8.28±0.17 eV.

Precision frequency measurements began immediately, with Jun Ye's laboratory at JILA making a direct comparison to a 87Sr optical atomic clock.

In addition to the capabilities of today's atomic clocks, such as satellite-based navigation or data transfer, its high precision will allow new applications inaccessible to other atomic clocks, such as relativistic geodesy, the search for topological dark matter,[63] or the determination of time variations of fundamental constants.

[65] The central idea is that the low energy is due to a fortuitous cancellation between strong nuclear and electromagnetic effects within the nucleus which are individually much stronger.

Any variation the fine-structure constant would affect the electromagnetic half of this balance, resulting in a proportionally very large change in the total transition energy.

[24][62] A change of even one part in 1018 could be detected by comparison with a conventional atomic clock (whose frequency would also be altered, but not nearly as much), so this measurement would be extraordinarily sensitive to any potential variation of the constant.