Microscopic Observation of Pauli Blocking in Degenerate Fermionic Lattice Gases

Abstract

The Pauli exclusion principle is one of the most fundamental manifestations of quantum statistics. Here, we report on its local observation in a spin-polarized degenerate gas of fermions in an optical lattice. We probe the gas with single-site resolution using a new generation quantum gas microscope avoiding the common problem of light induced losses. In the band insulating regime, we measure a strong local suppression of particle number fluctuations and a low local entropy per atom. Our work opens a new avenue for studying quantum correlations in fermionic quantum matter both in and out of equilibrium.Quantum statistics distinguishes between two fundamentally different kinds of particles: bosons, which condense into a single quantum state at zero temperature, and fermions, for which multiple occupancy of a single state is forbidden. As a result, identical fermions seem to repel each other, described by an effective Fermi pressure on a macroscopic level [1]. Microscopically, the Pauli blockade manifests itself in a strong suppression of density fluctuations [2,3] and in antibunching of densitydensity correlations [4][5][6][7][8][9]. Specifically, fermions in periodic potentials form a band insulating state with suppressed number fluctuations on each site when the chemical potential lies in the band gap. Measuring these number fluctuations can therefore be regarded as a direct probe of Pauli blocking.Local fluctuations in periodic potentials have been directly studied with ultracold bosonic atoms in optical lattices [10][11][12]. Some of these experiments featured siteresolved fluorescence detection with single atom sensitivity [13,14], which has proven to be a powerful method for probing quantum many-body systems. However, such quantum gas microscopy requires a specialized experimental setup with considerably increased technical complexity. In particular, the often used fermionic alkali atoms are difficult to laser cool, making the single-site and atom resolved detection even more challenging. First results have recently been reported on the imaging of single fermions in dilute thermal clouds [15][16][17][18]. However, microscopy of quantum degenerate fermions has so far remained out of reach.Here, we report on the site-resolved characterization of a spin-polarized degenerate Fermi gas in an optical lattice. In the band-insulating region, we measure a strong suppression of local atom number fluctuations, more than one order of magnitude below the Poisson limit expected for uncorrelated particles. Based on the measurement of the local occupation statistics, we reconstruct the spatial entropy distribution in the inhomogeneous samples. We obtained these results with a conceptionally novel quantum gas microscope based on an additional, dedicated optical lattice for detection. This provides high flexibility for future experiments and offers an alternative approach [19,20] to overcome the limitations due to parity detection [12,14].Our experiments started in a standard magneto-optical trap of 6 Li loaded from a Zeeman...

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