Shuying Chen

Abstract:

In quantum logic spectroscopy, internal transitions of trapped ions and molecules can be probed by measuring the motional displacement caused by an applied light field of variable frequency. This provides a solution to “needle-in-a-haystack” problems, such as the search for narrow clock transitions in highly charged ions, recently discussed by Chen []. The main bottleneck is the search speed over a frequency bandwidth, which can be increased by enhancing the sensitivity of displacement detection. In this work, we explore two complementary improvements: the use of squeezed motional states, explained using an analytical phase-space model, and optimal statistical postprocessing of data using a hypothesis-testing framework. We demonstrate that each method independently provides a substantial boost to search speed. Their combination effectively mitigates state preparation and measurement errors, improving the search speed by an order of magnitude and fully leveraging the quantum enhancement offered by squeezing.

Abstract:

The Ni^{12+} ion features an electronic transition with a natural width of only 8 mHz, allowing for a highly stable optical clock. We predict that the energy of this strongly forbidden 3s^{2}3p^{4} ^{3}P_{2}→3s^{2}3p^{4} ^{3}P_{0} electric quadrupole transition is 20 081(10)  cm^{-1}. For this, we use both a hybrid approach combining configuration interaction with a coupled-cluster method and a pure configuration interaction calculation for the complete 16-electron system, ensuring convergence. The resulting very small theoretical uncertainty of only 0.05% allowed us to find the transition experimentally in a few hours, yielding an energy of 20 078.984(10)  cm^{-1}. This level of agreement for a 16-electron system is unprecedented and qualifies our method for future calculations of many other complex atomic systems. While paving the way for a high-precision optical clock based on Ni^{12+}, our theory and code development will also enable better predictions for other highly charged ions and other complex atomic systems.

Abstract:

We measured the g-factor of the excited-state ^{3}P_{1} in Ca^{14+} ion to be g=1.499032(6) with a relative uncertainty of 4×10^{-6}. The magnetic field magnitude is derived from the Zeeman splitting of a Be^{+} ion, cotrapped in the same linear Paul trap as the highly charged Ca^{14+} ion. Furthermore, we experimentally determined the second-order Zeeman coefficient C_{2} of the ^{3}P_{0}-^{3}P_{1} clock transition. For the m_{J}=0→m_{J^{'}}=0 transition, we obtained C_{2}=0.39±0.04  Hz mT^{-2}, which is to our knowledge the smallest reported for any atomic transition to date. This confirms the predicted low sensitivity of highly charged ions to higher-order Zeeman effects, making them ideal candidates for high-precision optical clocks. Comparison of the experimental results with our state-of-the art electronic structure calculations shows good agreement and demonstrates the significance of the frequency-dependent Breit contribution, negative energy states, and QED effects on magnetic moments.

Abstract:

Nonlinearities in King plots (KP) of isotope shifts (IS) can reveal the existence of beyond-standard-model (BSM) interactions that couple electrons and neutrons. However, it is crucial to distinguish higher-order standard model (SM) effects from BSM physics. We measure the IS of the transitions ^{3}P_{0}→^{3}P_{1} in Ca^{14+} and ^{2}S_{1/2}→^{2}D_{5/2} in Ca^{+} with sub-Hz precision as well as the nuclear mass ratios with relative uncertainties below 4×10^{-11} for the five stable, even isotopes of calcium (^{40,42,44,46,48}Ca). Combined, these measurements yield a calcium KP nonlinearity with a significance of ∼10^{3}σ. Precision calculations show that the nonlinearity cannot be fully accounted for by the expected largest higher-order SM effect, the second-order mass shift, and identify the little-studied nuclear polarization as the only remaining SM contribution that may be large enough to explain it. Despite the observed nonlinearity, we improve existing KP-based constraints on a hypothetical Yukawa interaction for most of the new boson masses between 10  eV/c^{2} and 10^{7}  eV/c^{2}.