The Fermi Gas Microscope

New possibilities for quantum simulations of condensed matter models

C. S. Chiu, G. Ji, A. Mazurenko, D. Greif, M. Greiner

Fig. 1 | A densely populated two-dimensional square optical lattice in the image plane of the Fermi Gas Microscope. Click the image for a full-sized version.

We have built a second-generation quantum gas microscope for fermionic atoms. Our implementation is based on ultracold 6Li atoms trapped in an optical lattice with 569 nm spacing. We have demonstrated single-site resolved fluorescence imaging for these atoms.

Our imaging technique uses pulsed Raman sideband cooling in a mK deep optical lattice to overcome the strong recoil heating associated with the light mass of lithium. In combination with a gated intensified CCD camera, we are now routinely able to obtain images such as the one in Fig. 1. A preprint on how we create site-resolved images is available on the arXiv..

Recent Experiments

Experimental realization of a long-range antiferromagnet in the Hubbard model with ultracold atoms

Many exotic phenomena in strongly correlated electron systems emerge from the interplay between spin ordering and motional degrees of freedom. For example, doping an antiferromagnet is expected to give rise to interesting phases including pseudogap states, stripe-ordering and incommensurate spin order. Ultracold fermions in optical lattices offer the potential to answer open questions on the low-temperature regime of the doped Hubbard model, which is thought to capture essential aspects of the cuprate superconductor phase diagram but is numerically intractable in that parameter regime.

We have observed antiferromagnetic long-range order in a repulsively interacting Fermi gas of Li-6 atoms on a 2D square lattice containing about 80 sites. At our lowest temperature of T/t=0.25, the ordered state is directly detected from a peak in the spin structure factor and a diverging correlation length of the spin correlation function. When doping away from half-filling into a numerically intractable regime, we find that long-range order extends to doping concentrations of about 15%. Our results open the path for a controlled study of the low-temperature phase diagram of the Hubbard model.

Site-resolved observations of antiferromagnetic correlations in the Hubbard model

Quantum many-body systems exhibiting magnetic correlations underlie a wide variety of phenomena. High-temperature superconductivity, for example, can arise from the correlated motion of holes on an antiferromagnetic Mott insulator. Strongly correlated many-body systems can be realized using ultracold fermionic atoms in optical lattices with a tunability that is unparalleled in conventional solid-state systems.

Using our Fermi Gas Microscope with Li-6 atoms, we have observed antiferromagnetic correlations in a Hubbard-regime optical lattice with single-site resolution. Our detection technique relies on a selective spin-removal technique and subsequent site-resolved imaging. This allows measuring the spin correlation function, which is found to decay exponentially. The extraordinary detection capabilities of the microscope also allow us to study lattice loading dynamics affect and make comparisons to theory at an unprecedented level of detail. Our temperatures are the lowest reported in a Hubbard model system with cold atoms and approach the limits of available numerical techniques. Our results demonstrate that quantum gas microscopy is a powerful tool for studying fermionic quantum magnetism.

Site-resolved imaging of a fermionic Mott insulator

Conventional band theory predicts an insulating behavior of a solid if the freely moving electrons occupy every possible quantum state in the highest energy band, whereas the state is conducing otherwise. This simple picture of band insulating and conducting metallic states is altered in the presence of strong interactions, which can lead to an insulating behavior even in the presence of a half-filled energy band. These Mott insulators, named after the British physicist Sir Nevill F. Mott, are one of the conceptually simplest examples of many-body systems, where strong correlations lead to surprising phenomena.

We have observed fermionic Mott insulators, metals and band insulators with single-site resolution by trapping a repulsively interacting, two-component mixture of Li-6 atoms in a square optical lattice. We observe large, defect-free 2D Mott insulators containing more than 400 atoms with an average entropy per particle of 1.0kB.

The images above show single-shot picture of the atomic distribution for varying interactions U/t, along with the deconvolved images in the bottom row. The metallic state (left picture) is characterized by a large occupation variance, the band insulating region appears as a core of empty sites (middle picture) and the Mott insulator is signaled by an extended region of constant occupation with one particle per site and strongly reduced occupation variance (right picture and ring in middle picture).