IBM has made a significant advance on the road to larger and more powerful quantum computers by linking two such devices together and performing calculations beyond the capabilities of either alone. The result is a positive sign that the company’s bet on a modular approach to scaling up quantum computers is feasible.

Quantum computers promise to be able to solve certain problems much faster than conventional devices, but major hurdles remain, including making the computers large enough while reducing errors. Various research groups and companies are taking different approaches to…