Quantum computers and other devices hold incredible promise for enabling new types of sensors and communications, improving simulations of physical and chemical systems, and ushering in new levels of secure cryptography. While dangling such possibilities, quantum advances also pose something of a conundrum: Once quantum systems are big enough, standard computers will not be powerful enough to check the accuracy of the quantum devices. For the last three years and with funding from the Agile Science of Test and Evaluation program within the Air Force Office of Scientific Research (AFOSR), three Caltech professors and their colleague from Stanford University have been addressing the question of how to assess the performance of quantum systems.…