# Projects

## Research Topics and Interests

### Cold Atoms, Ions, and Molecules

One of the research topics in the group of Prof. Eric Hudson at the University of California, Los Angeles, is the study of cold atoms, ions, and molecules. Our experimental system is a hybrid atom-ion trap: We can trap singly-charged barium, ytterbium, and barium chloride ions in a radio-frequency (RF) (or Paul) trap. The ions can be overlapped with calcium atoms in a magneto-optical trap (MOT). Both the (atomic) ions and atoms can be laser cooled to a few millikelvin.

The experimental setup is an ideal tool for various experimental studies. First, it allows to study cold collisions and cold reactions between ions and atoms close to the absolute zero temperature. Second, the calcium MOT can be used to cool the internal degrees of freedom of molecular ions, in our case, barium chloride.

Our system has been extended by an integrated time-of-flight mass spectrometer (TOFMS), which required the development of electronics for operating both RF trap and TOFMS. We demonstrated an isotopic resolution of the TOFMS and showed that laser cooling the ion samples leads to a significant improvement of mass resolution and detection limit (Phys. Rev. Appl. 2, 034013 (2014) and Int. J. Mass Spectrom. 394, 1-8 (2016); see also laser-cooling-assisted mass spectrometry in the news).

### Thorium-229 Isomeric Transition

The second project in the group of Prof. Eric Hudson is the search for the thorium-229 isomeric transition. For almost 40 years, this low-energy nuclear transition has been known to exist. Yet, despite decades of searching, it has never been directly observed. If found, this transition could lead to an ultraprecise optical clock and precision tests of physics beyond the Standard Model.

Although originating from the nucleus, the transition shares the typical characteristics of atomic electronic transitions used in precision experiments such as optical clocks: It's wavelength is in the optical regime (more precisely, the vacuum ultraviolet (VUV) regime) as compared to all other known nuclear transitions, which have transitions at much higher energies. Further, the transition is expected to have a long lifetime of likely order hours. As a result, it becomes accessible to state-of-the-art atomic physics methods such as laser spectroscopy and renders itself inaccessible to nuclear physics techniques. The isomeric transition in thorium-229 is the only known nuclear transition with such properties.

Our approach is a direct search using thorium-doped crystals and spectroscopy of the isomeric state with VUV light. After illuminating the sample, we detect (single) photons, potentially from the decay of isomeric transition, with photo-multiplier tubes (PMTs).

In a first experiment, we used the Advanced Light Source (ALS) synchrotron at Lawrence Berkeley National Laborator (LBNL) as a tunable VUV light source (beamline 9.0.2, which is normally used for chemical dynamics experiments). The experiment excludes large portions of the wavelength–lifetime region of interest and can be used to guide future searches ( Phys. Rev. Lett. 114, 253001 (2015)).

As a next step, we collaborated briefly with the group of Cheuk-Yiu Ng at UC Davis, in particular Yih-Chung Chang, who are physical chemists and employ pulsed VUV lasers in the spectroscopy of molecular structure (amongst other applications). Subsequently, we returned to UCLA, where we built our own VUV laser system. This laser system which will increase our experimental sensitivity significantly over the ALS and allow to search a larger region including longer lifetimes. Currently, we can produce up to $35 \mu \mbox{J} / \mbox{pulse}$ at a wavelength of around $165 \mbox{nm}$.

While we (in the Hudson group) provide the atomic physics and laser spectroscopy know-how and background, this work relies on—and would not have been possible without—an interdisciplinary collaboration: The thorium-229 source material was provided by Saed Mirzadeh/Oak Ridge National Lab. The further chemical purification and growing of thorium-doped crystals was performed by A. Cassanho and H. P. Jenssen (AC Materials, Inc.). Last but not least, Eugene Tkalya (Lomonosov Moscow State University, Moscow) has the nuclear physics expertise and experience, which is indispensable for the design and understanding of the experiment.