  With the growing availability of experimental loophole-free Bell tests, it
has become possible to implement a new class of device-independent random
number generators whose output can be certified to be uniformly random without
requiring a detailed model of the quantum devices used. However, all of these
experiments require many input bits in order to certify a small number of
output bits, and it is an outstanding challenge to develop a system that
generates more randomness than is consumed. Here, we devise a
device-independent spot-checking protocol that consumes only uniform bits
without requiring any additional bits with a specific bias. Implemented with a
photonic loophole-free Bell test, we can produce 24% more certified output bits
(1,181,264,237) than consumed input bits (953,301,640). The experiment ran for
91.0 hours, creating randomness at an average rate of 3606 bits/s with a
soundness error bounded by $5.7\times 10^{-7}$ in the presence of classical
side information. Our system will allow for greater trust in public sources of
randomness, such as randomness beacons, and may one day enable high-quality
private sources of randomness as the device footprint shrinks.
