  A quantum random number generator (QRNG) can generate true randomness by
exploiting the fundamental indeterminism of quantum mechanics. Most approaches
to QRNG employ single-photon detection technologies and are limited in speed.
Here, we propose and experimentally demonstrate an ultrafast QRNG at a rate
over 6 Gb/s based on the quantum phase fluctuations of a laser operating near
threshold. Moreover, we consider a potential adversary who has partial
knowledge on the raw data and discuss how one can rigorously remove such
partial knowledge with post-processing. We quantify the quantum randomness
through min-entropy by modeling our system, and employ two extractors,
Trevisan's extractor and Toeplitz-hashing, to distill the randomness, which is
information-theoretically provable. The simplicity and high-speed of our
experimental setup show the feasibility of a robust, low-cost, high-speed QRNG.
