Researchers have detected the nuclear resonance of thorium-229 by measuring how much ultraviolet light a doped crystal absorbs. Earlier experiments watched the excited nuclei release light slowly after laser exposure. Direct absorption supplies a signal while excitation is happening, giving a solid-state nuclear clock a faster way to correct its laser frequency. [1] [2]
A nuclear clock would use an energy change inside an atomic nucleus as its frequency reference, rather than the electron transitions used by today's optical atomic clocks. Thorium-229 is unusual because its 8.4-electronvolt transition can be reached with 148.4-nanometre vacuum-ultraviolet light. In previous crystal experiments, pulsed sources spread their light across many frequencies and detection relied on fluorescence with a decay time of about 600 seconds. That delay leaves less time for comparing the clock laser with the nuclear reference. [1]
The new experiment concentrates sub-nanowatt continuous-wave light into the resonance instead. An infrared diode laser begins at 1,187 nanometres; three successive frequency-doubling stages bring it to 148.4 nanometres. The beam passes through a calcium-fluoride crystal containing thorium-229, and a detector behind the crystal measures the transmitted light. The system alternates between an on-resonance frequency and one shifted away from resonance, so the difference isolates nuclear absorption from slow changes in laser power. [1] [3]
The first absorption scans alternated the two frequencies every four seconds and averaged each point for 320 seconds; those recordings were typically five times shorter than the fluorescence measurements. With the laser locked to a high-finesse reference cavity, the team then modulated it at 10 hertz and averaged each point for three seconds. That reduced the detection cycle by two orders of magnitude compared with fluorescence and resolved a 91-kilohertz-wide line that can generate the error signal needed to steer a clock laser. [1]
The absorption spectrum also separated how thorium sits in the crystal. Five lines came from a paired-thorium defect called the D-centre, while a broader line came from a more symmetric O-centre. The measured frequency difference between them was 3.99 megahertz; a density-functional calculation gave 3.60 megahertz within its stated uncertainty. The O-centre's static electric-field gradient was below 0.1 volt per square ångström, compared with roughly 100 for the D-centre. The authors infer that this symmetry may make the resonance less sensitive to lattice spacing, but temperature and strain tests are still needed. [1] [2]
This is a spectroscopy and feedback demonstration, not an operating nuclear clock or a measured clock-accuracy result. It changes the practical route by replacing a wait for delayed fluorescence with a continuous transmission measurement that can guide the laser in real time. The next decisive test is a long-running frequency lock that reports stability and systematic shifts while crystal temperature, thorium concentration and mechanical strain are varied, followed by reproduction in another laboratory. [1]