Sensing T-violating nuclear moments of paramagnetic ions in crystals

Abstract

Precision measurements of time-reversal (T) symmetry violating moments probe physics beyond the Standard Model. We show that precision spectroscopy of paramagnetic lanthanide and actinide ions doped into noncentrosymmetric crystals offers a promising platform for extending the sensitivity of searches for T-violation in nuclear physics. The unpaired valence electron in these ions allows the engineering of highly-coherent hyperfine transitions that are insensitive to magnetic fields, yet highly sensitive to new physics. These systems also provide other advantages for new physics searches, including large numbers of ions that can be measured in doped crystals, strong electric polarization of the ions by the crystal fields, enhancement of T-violating nuclear moments in nonspherical nuclei, and accurate comagnetometers generated by crystal symmetry. We estimate the new physics sensitivity of these proposed systems to be two orders of magnitude better than existing constraints.

Figures (2)

• Figure 1: Schematic of the energy level structure of paramagnetic ions in YSO. The degeneracy of electron spin sublevels in the $Z$ and $Y$ electronic states is lifted by the interactions with the crystal. An optical transition between the two electronic states can be used for controlling and probing the ions. Each of the energy levels $Z_i, Y_j$ are doubly-degenerate Kramers pairs. Within each of these is a manifold of hyperfine states.
• Figure 2: Example NTSC transitions for site 1 ions of $^{167}$Er:YSO. Each column represents a transition ${\left| {i} \right\rangle}\to{\left| {f} \right\rangle}$, with the hyperfine sublevels labeled in the order of low to high energy. The magnitude of the magnetic field is varied along the direction of the NTSC magnetic field for each transition ($\hat{r}_k$ for transitions labeled by $k\in 0, 1, 2, 3$). The top row shows the expectation values of the electron spin projection $\hat{S}=\vec{S} / |\vec{S}|$ and the nuclear spin projection $\hat{I}=\vec{I} / |\vec{I}|$ of initial and final states at the NTSC magnetic fields in the $x-y$ plane. The electron spins of the two states have nearly identical expectation values which ensures that both states have identical magnetic moments, while the nuclear spins in these states have different expectation values. The magnetic field sensitivity, the NSM sensitivity, and the MQM sensitivity are shown as a function of the magnetic field. The NTSC magnetic fields are labeled as orange dashed lines, and the NSM and MQM sensitivities at the NTSC magnetic field are denoted by red dots.