  The speed of a quantum random number generator is essential for practical
applications, such as high-speed quantum key distribution systems. Here, we
push the speed of a quantum random number generator to 68 Gbps by operating a
laser around its threshold level. To achieve the rate, not only high-speed
photodetector and high sampling rate are needed, but also a very stable
interferometer is required. A practical interferometer with active feedback
instead of common temperature control is developed to meet requirement of
stability. Phase fluctuations of the laser are measured by the interferometer
with a photodetector, and then digitalized to raw random numbers with a rate of
80 Gbps. The min-entropy of the raw data is evaluated by modeling the system
and is used to quantify the quantum randomness of the raw data. The bias of the
raw data caused by other signals, such as classical and detection noises, can
be removed by Toeplitz-matrix hashing randomness extraction. The final random
numbers can pass through the standard randomness tests. Our demonstration shows
that high-speed quantum random number generators are ready for practical usage.
