Tools for the task at hand
Referring to the machine in his laboratory, Mikhail Lukin, one of the leaders of the Harvard team, told Live Science that "It's basically a quantum simulator."
That means the computer is built for a specific task: to study the evolution of quantum systems. It won't be breaking encryption codes on the world's banks, finding the highest mountain in a mountain range or pulling off any of the other tasks for which general quantum computers are suited.
Instead, the Harvard and UMD machines are really good at solving a particular kind of problem: If a complicated quantum system starts in one state, how will it move and evolve?
It's a narrow question, but in solving it, the researchers are developing technologies and making new discoveries in physics that will allow for even more complicated computers, which will pull off even more impressive tasks.