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Single-atom-resolved detection and manipulation of strongly correlated fermions in an optical lattice

Final Report Summary - FERMILATT (Single-atom-resolved detection and manipulation of strongly correlated fermions in an optical lattice)

The goal of the project was to study strongly correlated fermionic quantum systems in an optical lattice with single-atom and single-site resolution using a quantum-gas microscopes setup. One of the key goals of the project was the realisation of single-site-resolved detection of fermionic potassium-40 atoms in a quantum-gas microscope. Our project team was amongst the first groups worldwide to report these achievements and our paper [Nature Physics 11, 738 (2015)] has achieved worldwide attention. The fermionic quantum-gas microscopes were listed as “Highlights of the Year 2015” by the American Physical Society.

To achieve single-site-resolved detection of individual atoms on the lattice, it is desirable to maximize the fluorescence yield while at the same time maintaining a negligible particle loss rate and preventing the atoms from hopping between lattice sites. These conditions can be effciently achieved by simultaneously laser cooling the atoms to sub-Doppler temperatures while detecting the fluorescence photons emitted during this process. However, cooling of fermionic alkaline atoms in optical lattices has proven is challenging, as their low mass and small excited-state hyperfine splitting make it more difficult to obtain low temperatures using the standard technique of polarization-gradient cooling.

We overcame this difficulty by using a new form of cooling that has never been used in optical lattice experiments before, which is electromagnetically-induced transparency (EIT) cooling. Our cooling and imaging technique [see details in Nature Physics 11, 738 (2015)] works so well, such that we could detect on average 1000 fluorescence photons from a single atom within 1.5 s, while keeping it close to the vibrational ground state of the optical lattice. The fluorescence images of individual atoms showed that the point spread function of single atoms matches the expected diffraction limited resolution of the imaging system. We studied the characteristics of EIT cooling in our experiment by measuring the cloud width, cooling rates, lifetimes, fluorescence yield and loss and hopping rates, as a function of cooling parameters. We also discovered a new method of gray molasses cooling on the D2-line, which will be published likely in December 2016.

In summary, we have achieved a major milestone by achieving single-site-resolved detection of fermions in an optical lattice, a results that received a lot of international attention. Our single-site-resolved detection of fermionic atoms in an optical lattice will open the path to the study of strongly correlated fermionic quantum systems. Work on the experimental setup is ongoing after the end of the ERC grant. We are currently working to realize much colder samples and lower entropies to realize, e.g. fermionic Mott insulators. The quantum-gas microscope will enable manipulation an observation of these systems with single-site resolution, allowing for example the study of out-of-equilibrium dynamics of many-body fermionic quantum systems.