Catani et al. exchange entropy among quantum gases using element-selective trapping potentials. (a) The schematic diagram shows an admixture of two quantum gases, in which the refrigerator gas (red) and target gas (tan) are held in the same confining potential. Entropy can be transferred from the target gas to the refrigerator gas in two equivalent ways. (b) One approach is to compress the target gas selectively. Catani et al. accomplish this using element-specific optical forces, while, in the figure, this is shown schematically by the displacement of an element-specific barrier. (c) A second approach is to expand the refrigerator gas into a larger volume while keeping the volume of the target gas constant, for example, by using an element-specific barrier. While the final temperatures following these two procedures are different, the distribution of entropy between the two gases may be the same. (d) Alternate approaches are being considered where system parameters are tuned spatially so that a quantum gas achieves a gapped phase (shown schematically at left) in one spatial region, and a gapless phase (at right) in a neighboring region into which the entropy will be concentrated. Here, the statistical occupancy of different states is indicated by the size of the shaded circles. Either scheme may allow the preparation of target gases at entropy densities sufficiently low to realize new types of quantum matter.