Cold Molecules for Fundamental Physics

Project Information

CoMoFun

Grant agreement ID: 949119

DOI

10.3030/949119

EC signature date 25 August 2020

Start date 1 May 2021

End date 30 April 2027

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 875 750,00

EU contribution

€ 1 875 750,00

Coordinated by

IMPERIAL COLLEGE OF SCIENCE TECHNOLOGY AND MEDICINE
United Kingdom

Periodic Reporting for period 3 - CoMoFun (Cold Molecules for Fundamental Physics)

Reporting period: 2023-12-01 to 2025-05-31

Summary of the context and overall objectives of the project

The development of quantum physics was one of the main driving forces for technological and societal transformation in the twentieth century. The precise control of atoms and molecules has been essential in developing quantum physics. Particularly, the ability to use lasers to cool atoms in a magneto-optical trap (MOT) has revolutionized modern physics. A MOT uses precisely tuned lasers and a magnetic field to cool atoms and trap them. It has enabled the invention of precise instruments, such as atomic clocks, magnetometers, gravimeters, and accelerometers. Precise atomic clocks are key to global navigation satellite systems such as GPS, GLONASS, BeiDou, or Galileo. Atomic quantum sensors are an emerging technology with unparalleled accuracy and precision. The magneto-optical trap has also enabled new fundamental research with unprecedented precision and the study of matter dominated by quantum effects. However, there is still potential to push the boundaries of science and technology: by using ultracold molecules. CoMoFun aims to do just this, creating a high-density ultracold gas of polar molecules by laser cooling to build a new platform for fundamental research. A high-density ultracold gas of polar molecules has a wide range of new applications. It can be used to study new phases of matter, to test fundamental physics and to store and process quantum information efficiently. An array of polar molecules, all interacting with each other via controllable and strong interactions, can serve as a universal simulator for more complex quantum systems that cannot be modelled by a computer. Simulating such strongly interacting many-body systems from the bottom up will aid the understanding of fascinating phenomena such as high-temperature superconductivity and exotic forms of magnetism. Moreover, molecules exhibit ultra-narrow transitions in the mid-infrared spectral range, offering potential applications in metrology. Their heightened sensitivity to direct current and microwave fields positions them as ideal sensors for various new applications.

Recently, it has become possible to make a MOT of molecules. However, the density of the molecules is far too low for most applications. CoMoFun will increase the density by five orders of magnitude by laser-cooling stable and deeply bound aluminium monofluoride molecules. The high density provides an excellent starting point to investigate evaporative cooling to quantum degeneracy. The molecules can then be arranged in a regular array by loading them into a trap formed by interfering laser beams. This instrument can then be used for precision measurements and applications in quantum information and simulation, to realize the full potential of molecular MOT.

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

Direct molecular cooling experiments typically begin with developing a source for the molecules. Contrary to atoms, diatomic, polar molecules are not readily available and must be produced. This is achieved by empirical, not very well-understood methods. CoMoFun studied the formation process of AlF in detail and tailored the chemical reactions to maximise the flux. This is key for a high-density molecular MOT. We designed, built, and characterised cryogenic molecular beam sources and compared the flux to other species. This way we could understand why some molecules are produced with higher efficiency than others. We could show that the production efficiency is tenfold higher for AlF, making it the most intense cryogenic buffer gas beam to date. In addition, we developed a unique cw molecular beam of AlF. Typical molecular beam sources produce short pulses of molecules severely limiting the number of molecules loaded into a trap. The new cw source uses in-depth knowledge of the chemical reactions at high temperatures resulting in a molecular beam over 100 times brighter than the pulsed beam. Further characterisation and cooling to reduce the forward velocity is, however, required.

An intense molecular beam is an ideal starting point for further slowing and cooling experiments. The next step on the journey to a high-density MOT is an in-depth understanding of the laser cooling process. However, experiments are challenging because AlF’s main laser cooling transition lies in the deep UV range of the spectrum near 228 nm and the large number of levels involved in the cooling process complicates the theoretical understanding. We characterised new custom-made optics and found creative solutions to the challenges that arise from producing high-power deep UV laser radiation. The first experiments demonstrated the large optical force that can be exerted on AlF by comparing these results to a detailed theoretical model we gained a crucial understanding of the cooling process. We proceeded to the next phase: building the MOT apparatus.

However, post-COVID supply chain disruptions caused significant delays in reaching the next objectives. We pivoted our research and started to develop our own laser systems and tested their stability, the custom vacuum chamber, deep UV optics and windows by loading a Cd atomic MOT. Cd is the ideal test species for AlF because it has a similar electronic structure and transition wavelength while requiring fewer lasers. Experiences gained from the loading and cooling of Cd will be invaluable for future molecular MOT experiments. We demonstrated the largest Cd MOT to date, which by itself is a major achievement. In addition, we resolved discrepancies in the literature concerning the isotope shifts and established Cd as a new candidate for precision searches of new physics beyond the Standard Model of Particle Physics. To demonstrate how we can push the current limits of laser technology we also performed high-resolution spectroscopy of Zn at 214 nm.

Finally, we studied the possibility of narrow-line cooling of AlF molecules through careful spectroscopic measurements of the electronic states in AlF. Narrow-line cooling is commonly used for alkaline earth atoms to reach very high densities at low temperatures. Adapting this technique to diatomic molecules to improve their in-trap density will have a major impact in the field.

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)

Laser cooling experiments in the deep UV spectrum are challenging and rare. Demonstrating that it is indeed feasible to perform cutting-edge research in this domain goes beyond the current state of the art. The development of new laser sources, custom optics, and the detailed analysis of UV-induced damage will benefit a much wider research community and enable new research directions in fields, such as atomic, molecular, and optical physics, quantum technology, biology, medicine, and environmental science. CoMoFun is a high-risk, high-reward project that will transform and expand our knowledge of laser cooling of molecules and establish a new class of molecules for next-generation precision experiments. In the upcoming year, we progress towards loading an AlF MOT and compare the loading efficiency to Cd. We are confident that the density of the trapped molecules will be significantly higher as compared to other molecular species. This will mark a significant milestone and will go well beyond the current state of the art. We anticipate that after cooling in the MOT the molecules can be loaded efficiently into a conservative, deep dipole trap where we aim to study the collisions between molecules. This would open a new way to cool the molecules to quantum degeneracy via evaporative cooling.