When it comes to furthering our overall understanding of the physical world, ultracold quantum gases are awfully promising. As the famous physicist Richard Feynman argued, to fully understand nature, we need quantum means of simulation and computation. Ultracold atomic systems have, in the last 30 years, proven to be amazing quantum simulators. The number of applications for these systems as such simulators is nothing short of overwhelming, ranging from engineering artificial crystals to providing new platforms for quantum computing. In its brief history, ultracold atomic experimental research has enhanced physicists’ understanding of a truly vast array of important phenomena.
One of the revelations of quantum mechanics is that any object can be seen as a wave (even you!) when an appropriate experimental test is used. Properties of these co-called “matter waves” depend on their temperature; at large temperatures they have short wavelengths and look and behave particlelike because all the peaks and valleys are so close together that they cannot be told apart. If we lower temperature to much less than a single kelvin, the wave nature of matter becomes more pronounced and wavelike behaviors more important. What happens then with a large collection of very cold atoms that behave like a large collection of waves? They can all align and overlap to form a single wave, something that was historically called a “macroscopic wave function.” Such a system—a condensate in physics parlance—is a fundamentally quantum state of matter.