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.

Red cylindrical crystal of calcium fluoride

Uranium doped CaF2 single crystal. The red part of the crystal is where the Uranium was integrated into the calcium fluoride lattice. (crystal.jpg)

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.

A cubic lattice of calcium fluoride with dopants

Energetically most favourable charge compensation scheme predicted by DFT (density functional theory) calculations. Th4+ ion replaces Ca2+ and two extra F- are inserted into the lattice during crystallization

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.

Energy levels scheme of Thorium-229 nucleus

The energy level scheme for a solid-state nuclear clock based on 229Th:CaF2. The VUV laser couples the ground and isomer nuclear state which have different nuclear spin. The radiofrequency pulses are used for non-destructive readout of the population in the ground and isomer nuclear state

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.

A calcium fluoride crystal inside the solenoid coil

llustration of the “core” of the solid-state nuclear clock. The cylindrically shaped 229Th:CaF2 crystal is put into a solenoid NQR detection coil tuned to 300 MHz and 550 MHz. The 150 nm laser is used to generate the nuclear transition from ground to isomer state. The NQRS is used to readout the nuclear state population

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.