Ultracold mercury for a measurement of the EDM

Project Information

quMercury

Grant agreement ID: 757386

DOI

10.3030/757386

Project closed

EC signature date 19 February 2018

Start date 1 April 2018

End date 31 March 2025

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 939 263,00

EU contribution

€ 1 939 263,00

Coordinated by

RHEINISCHE FRIEDRICH-WILHELMS-UNIVERSITAT BONN
Germany

Periodic Reporting for period 5 - quMercury (Ultracold mercury for a measurement of the EDM)

Reporting period: 2024-04-01 to 2025-03-31

Summary of the context and overall objectives of the project

One of the most fundamental questions in contemporary physics is: "Why does the Universe contain matter?" or, more specifically: "Why does the Universe contain matter, but no antimatter?". The current Standard Model in physics is unable to explain the observed asymmetry between matter and antimatter, but there are stong indications that this asymmetry is related to a quantity called electric dipole moment (EDM). This is a tiny deviation in the shape of elementary particles from being perfectly spherical. Since more than 50 years, generations of researches are searching for a non-zero EDM. In this project, we take a new approach by using ultracold atoms, cooled by lasers to a millionth of a degree above absolute zero. In such a scenario, phenomena from quantum mechanics might allow us to boost measurement sensitivity.

Within the quMercury project, we have laid a solid foundation for EDM measurements based on laser-cooled mercury atoms. We have established magneto-optical trapping at high phase-space density and efficient transfer into an optical dipole trap. We have performed isotope shift spectroscopy to constrain the shape of the relevant nucleus, Hg-199. We are developing deep-UV laser systems for improved detection. Further, researchers of the quMercury project is involved in the panEDM collaboration, where contributions to the Hg magnetometer are made. Further topics include spectroscopy of xenon, which bear many similarities to mercury, and the development of precision metrology based on optical resonators.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

RP1: 01.04.2018 - 30.09.2019
We have started to set up the laboratory, including massive optical tables with a weight of many tons to provide a stable environment, we have set up the first laser systems and electronics, and designed the vacuum chamber in which the experiments will take place. Further, we have visited almost all other groups in Europe working on complementary approaches to measure the EDM, and organized a symposium on the occasion of the opening of the lab. Hired two PhD students and a number of internship students

RP2: 01.10.2019 - 31.03.2021
Our first magneto-optical trap of mercury, followed by detailed characterization and improvement of experimental parameters (Phys. Rev. A 105, 033106 (2022)). Set up an ultrastable cavity as frequency reference of the 254-nm laser.

RP3: 01.04.2021 - 30.09.2022
First optical dipole trapping of laser-cooled atoms, but the trap depth is not sufficiently deep to provide stable trapping. A new laser system is installed to provide sufficient trap depth. Also, a new experimental control system allows for high-resolution careful adjustment of experimental parameters. We commence isotope shift spoctroscopy on the 254-nm transition with unprecedented precision (1/1000 of the natural linewidth).

RP4: 01.10.2022 - 31.03.2024
We continue the isotope shift measurements and data evaluation on two further transitions near 313 nm, and prepare for a fourth set of measurements near 436 nm. To prepare for the dipole trapping and evaporative cooling experiments, we set up an entirely new vacuum chamber, explore new vacuum windows and sealing technology, refurbish the oven, and re-build the entire optical pathway.

RP5: 01.04.2024 - 31.03.2025
The focus shifted back to the optical dipole trapping of laser-cooled mercury atoms. Here, a new high-power laser system has been installed, with a new optical pathway for switching and control of the trap. A second trap axis has been added for axial confinement. Atoms have successfully been trapped in this crossed-beam dipole trap, and measurements of thermalization rates, which return information about scattering properties, have been initiated. Secondly, the setup of the 185-nm laser system been started. Thus far, a frequency-quadrupled system near 215 nm has been installed, a pump laser near 1300 nm has been installed, and the resonant sum-frequency-mixing stage has been set up.

Overview of results:
Construction of a laser-cooling experiment for mercury (described in various master and PhD theses), improved magneto-optical trapping (Phys. Rev. A 105, 033106 (2022)) and dipole trap loading (publication in preparation) of Hg atoms. Isotope shift spectroscopy (publication in preparation) to constrain nuclear parameters. Development of lasers in the range of 213 nm via two consecutive SHG stages (Phys. Rev. A 109, 012806 (2024), further manuscripts currently under review) and at 185 nm. Out of technology development performed in this project, ring laser gyroscopes emerged as a new topic with potential applications to search for physics beyond the standard model.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The project progressed well beyond the state of the art along various directions:

Laser cooling of mercury atoms: We have constructed the first experiment world-wide that aims at quantum degeneracy in the heaviest stable element that can be laser-cooled, mercury. This will be one of the first experiments using laser-cooled atoms or molecules for EDM measurements. Our magneto-optical trap exceeds all previous works by a large margin in terms of atom number and phase space density, providing a solid basis for experiments.

UV laser technology: Essentially all optical transitions in mercury are in the ultraviolet (UV) regime. Consequently, the project is pushing the state of the art in laser development. At the beginning of the project, our 254-nm system likely was the most powerful system, worldwide, in this wavelength range. We have developed a compact system at 310 nm with large tunability, set up a system at 214 nm, and are developing a system at 185 nm. This development gave rise to a new European consortium, UVQuanT.

Nuclear deformations studied from isotope shift spectroscopy: Nuclear deformations provide a large amplification factor of the nuclear EDM, and careful assessment of nuclear deformations is a prerequisite to interpret the measured EDM of a nucleus. Consequently, we performed isotope shift spectroscopy on various transitions. Thus far, this careful analysis was available only for ytterbium through a world-wide effort, and is becoming available for calcium as well. Investigating the heavy elements with their soft nuclei aids in the validation of nuclear models.

The neutron EDM: There are various systems under investigation to measure the electric dipole moment (EDM) of the neutron: the neutron itself, the Hg-199 nucleus, and the Xe-129 nucleus. Previously, these three approaches had no overlap. Within this project, we found intimate connections between these three approaches, all of which have representations within Europe (neutron EDM: nEDM collaboration at PSI and panEDM collaboration at ILL Grenoble; Xe-129 EDM: Heidelberg; Hg-199 EDM: Bonn). The quMercury project is now closely affiliated with all of these four initiatives, and even part of the panEDM collaboration. Various workshops have already emerged from this connection, master students have been supervised jointly, first joint proposals have been submitted, and further proposals are in preparation.

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