Mankind has long sought to control the properties of matter. Dating back millennia, alchemists have attempted, unsuccessfully, to transmute base metals into gold. I will discuss a modern analog of this problem: the quest to develop materials with emergent quantum mechanical properties that can be manipulated on demand. We focus on van der Waals materials, which can be mechanically isolated down to atomic monolayers and then stacked atop one another to create designer quantum matter. In particular, stacking graphene sheets with carefully controlled orientation can lead to a stunning array of new electronic properties. Whereas a single sheet of graphene behaves as a metal, under suitable conditions these all-carbon multilayer structures can be transformed into superconductors, ferromagnets, topological insulators, and more. We can further switch between these different quantum states of matter as desired using control knobs such as charge doping, electric and magnetic fields, pressure, and strain. These designer two-dimensional structures thus enable an exciting means of performing “quantum alchemy,” in which new quantum states can be systematically engineered and tailored.