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Science

Simulating transport with cold atoms

Jelena Stajic

Published Jan 24, 2019• 0 citations
Publisher

Abstract

Quantum Simulation Much can be learned about the nature of a solid from how charge and spin propagate through it. Transport experiments can also be performed in quantum simulators such as cold atom systems, in which individual atoms can be imaged using quantum microscopes. Now, two groups have investigated transport in the so-called Fermi-Hubbard model using a two-dimensional optical lattice filled with one fermionic atom per site (see the Perspective by Brantut). Moving away from half-filling to enable charge transport, Brown et al. found that the resistivity had a linear temperature dependence, not unlike that seen in the strange metal phase of cuprate superconductors. In a complementary study on spin transport, Nichols et al. observed spin diffusion driven by superexchange coupling. Science , this issue p. [379][1], p. [383][2]; see also p. [344][3] [1]: /lookup/doi/10.1126/science.aat4134 [2]: /lookup/doi/10.1126/science.aat4387 [3]: /lookup/doi/10.1126/science.aaw1326

DOI
10.1126/science.363.6425.360-b

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Quantum Simulation
Much can be learned about the nature of a solid from how charge and spin propagate through it. Transport experiments can also be performed in quantum simulators such as cold atom systems, in which individual atoms can be imaged using quantum microscopes. Now, two groups have investigated transport in the so-called Fermi-Hubbard model using a two-dimensional optical lattice filled with one fermionic atom per site (see the Perspective by Brantut). Moving away from half-filling to enable charge transport, Brown et al. found that the resistivity had a linear temperature dependence, not unlike that seen in the strange metal phase of cuprate superconductors. In a complementary study on spin transport, Nichols et al. observed spin diffusion driven by superexchange coupling.
Science , this issue p. 379, p. 383; see also p. 344
Quantum Simulation Two superconductors connected by a bridge made out of nonsuperconducting material form a so-called Josephson junction (see the Perspective by Belzig). Valtolina et al. replaced the superconductors with two reservoirs of a superfluid Fermi gas and connected them by a weak link to allow atoms to move from one side to the other. Then they made one reservoir more populated than the other and studied the ensuing dynamics as a function of interaction strength between the atoms. In a related experiment, Husmann et al. kept the interaction strength at its maximum, but varied the temperature and the properties of the link. As temperature increased, the superfluid disappeared and thermal transport took over. Science , this issue p. [1498][1], p. [1505][2]; see also p. [1470][3] [1]: /lookup/doi/10.1126/science.aac9584 [2]: /lookup/doi/10.1126/science.aac9725 [3]: /lookup/doi/10.1126/science.aad6876
Quantum Magnetism Strongly interacting electrons lined up along a string can experience the so-called spin-charge separation, where the electrons “split” into effective carriers of spin and charge, which then move independently. This phenomenon has been observed, somewhat indirectly, in solids. Hilker et al. show spin-charge separation in a direct way by using a one-dimensional (1D) array of cold atoms, playing the role of electrons, whose degrees of freedom of spin and charge can be monitored using a fermionic quantum gas microscope. Empty sites in the 1D lattice moved freely without disturbing the underlying antiferromagnetic order. Science , this issue p. [484][1] [1]: /lookup/doi/10.1126/science.aam8990
Quantum Simulation Studying topological matter in cold-atom systems may bring fresh insights, thanks to the intrinsic purity and controllability of this experimental setting. However, the necessary spin-orbit coupling can be tricky to engineer. Wu et al. conceived and experimentally demonstrated a simple scheme that involves only a single laser source and can be continuously tuned between one- and two-dimensional spin-orbit coupling (see the Perspective by Aidelsburger). Although this experiment used bosonic atoms, it is expected that the setup would also work for fermions. Science , this issue p. [83][1]; see also p. [35][2] [1]: /lookup/doi/10.1126/science.aaf6689 [2]: /lookup/doi/10.1126/science.aah7087
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Quantum Simulation One of the simplest models of interacting fermions on a two-dimensional (2D) lattice—the Hubbard model—becomes too tricky to simulate on classical computers as the density of empty lattice sites (holes) increases. Chiu et al. used a quantum microscope to take snapshots of thousands of realizations of the 2D Hubbard model in an optical lattice filled with fermionic lithium atoms at varying hole densities (see the Perspective by Schauss). The authors used pattern recognition algorithms to analyze the images, in which each lattice site was individually resolved. Comparing these patterns to the predictions of several theoretical models, they found the most consistency with the so-called geometric string model. Science , this issue p. [251][1]; see also p. [218][2] [1]: /lookup/doi/10.1126/science.aav3587 [2]: /lookup/doi/10.1126/science.aax6486
These lecture notes review recent progress in studying the Fermi-Hubbard model using ultracold gases in optical lattices. We focus on results from quantum gas microscope experiments that have allowed site-resolved measurements of charge and spin correlations in half-filled and doped Hubbard systems, as well as direct imaging of various types of polaronic quasiparticles. We also review experiments exploring dynamical properties of the Hubbard model through transport and spectroscopy. Moving beyond the plain-vanilla square-lattice Hubbard model, we present more recent work exploring Hubbard systems with novel lattice geometries and long-range interactions that stabilize new phases. Finally, we discuss the realization of entropy distribution protocols to cool these systems to ultralow temperatures where comparison to unbiased numerics is no longer possible.