Highly charged ions for atomic clocks and search for variation of the fine structure constant

Quadruply Ionized Barium as a Candidate for a High-Accuracy Optical Clock

We identify Ba^{4+} (Te-like) as a promising candidate for a high-accuracy optical clock. The lowest-lying electronic states are part of a ^{3}P_{J} fine structure manifold with anomalous energy ordering, being nonmonotonic in J. We propose a clock based on the 338.8 THz electric quadrupole transition between the ground (^{3}P_{2}) and first-excited (^{3}P_{0}) electronic states. We perform relativistic many-body calculations to determine relevant properties of this ion. The lifetime of the excited clock state is found to be several seconds, accommodating low statistical uncertainty with a single ion for practical averaging times. The differential static scalar polarizability is found to be small and negative, providing suppressed sensitivity to blackbody radiation while simultaneously allowing cancellation of Stark and excess micromotion shifts. With the exception of Hg^{+} and Yb^{+}, sensitivity to variation of the fine structure constant is greater than other optical clocks thus far demonstrated.

Detection of metastable electronic states by Penning trap mass spectrometry

State-of-the-art optical clocks¹ achieve precisions of 10^-18 or better using ensembles of atoms in optical lattices^2,3 or individual ions in radio-frequency traps^4,5. Promising candidates for use in atomic clocks are highly charged ions⁶ (HCIs) and nuclear transitions⁷, which are largely insensitive to external perturbations and reach wavelengths beyond the optical range⁸ that are accessible to frequency combs⁹. However, insufficiently accurate atomic structure calculations hinder the identification of suitable transitions in HCIs. Here we report the observation of a long-lived metastable electronic state in an HCI by measuring the mass difference between the ground and excited states in rhenium, providing a non-destructive, direct determination of an electronic excitation energy. The result is in agreement with advanced calculations. We use the high-precision Penning trap mass spectrometer PENTATRAP to measure the cyclotron frequency ratio of the ground state to the metastable state of the ion with a precision of 10^-11-an improvement by a factor of ten compared with previous measurements^10,11. With a lifetime of about 130 days, the potential soft-X-ray frequency reference at 4.96 × 10¹⁶ hertz (corresponding to a transition energy of 202 electronvolts) has a linewidth of only 5 × 10^-8 hertz and one of the highest electronic quality factors (10²⁴) measured experimentally so far. The low uncertainty of our method will enable searches for further soft-X-ray clock transitions^8,12 in HCIs, which are required for precision studies of fundamental physics⁶.