Readout scheme for solid-state nuclear clock
Atomic clocks operate by driving transitions between various states of valence electrons. This requires atoms to be isolated and trapped in optical or electric traps. Nuclear clocks will use nuclear transitions. Since the nucleus is largely unaffected by the valence electrons, it is no longer necessary to use isolated atoms. In our laboratory we are developing a nuclear clock by doping the 229Th into a VUV transparent CaF2 (Calcium Fluoride). The complex vacuum apparatus required for trapping and cooling the atoms can be replaced by a single crystal doped with 229Th atoms.
In the solid-state nuclear clock the the 229Th ions are in the 4+ charged state and have no valence electrons. This makes the conventional electron shelving readout schemes not applicable to ions doped into a CaF2 crystal.
The goal of CRYSTALCLOCK project is to develop an alternative readout scheme. If 229Th atoms are doped into the CaF2 crystal, the electric field gradient splits the nuclear states into various quadrupole levels.
The ground and isomeric states have different nuclear spin Igr=5/2 and Iis=3/2, respectively. Nuclear quadrupole resonance spectroscopy (NQRS) can measure the nuclear quadrupole splitting, and this splitting dramatically changes when the nucleus is excited. In this way the NQRS can be used to measure the population of the nuclei that were addressed by the nuclear clock laser.
To demonstrate the feasibility of the concept we have grown a Neptunium-237 doped CaF2. The Mössbauer spectrum was measured in ActUsLab, JRC, Karlsruhe.
We are developing the Thorium-229 dedicated cryogenic NQR spectrometer in Atominstitut, TU Wien.