Real-time high-speed physical random numbers are crucial for a broad spectrum of applications in cryptography, communications as well as numerical computations and simulations.Chaotic laser is promising to construct high-speed physical random numbers in real time benefitting from its complex nonlinear dynamics.However,the real-time generation rate of physical random numbers by using single-bit extraction is confronted with a bottleneck because of the bandwidth limitation caused by laser relaxation,which dominates the laser chaos and then limits the effective bandwidth only to a few GHz.Although some bandwidth-enhanced methods have been proposed to increase the single-bit generation rate, the potential is very limited,and meanwhile the defects of system complexity will be introduced.An alternative method is to construct high-speed physical random numbers by using the multi-bit extraction.In this method,each sampling point is converted to N digital bits by using multi-bit analog-to-digital converter (ADC) and their M(M 6 N) least significant bits are retained as an output of random bits,where N and M are the numbers of ADC bits and retained bits,respectively.The generation rate of random numbers is thus equal to M times sampling rate and can be greatly increased.Whereas,in the multi-bit extraction demonstrations,the intensity output of chaotic laser is usually digitized by the commercial oscilloscope and then processed with least-significant-bit retention followed by other postprocessing methods such as derivative,exclusive-OR,and bit-order reversal.These followed post-processing operations have to be implemented off-line and thus cannot support the real-time generation of random numbers.Resultantly,it is still an ongoing challenge to develop high-speed generation schemes of physical random numbers with the capability of real-time output.In this paper,a real-time high-speed generation method of physical random numbers by using multi-bit quantization of chaotic laser is proposed and demonstrated experimentally.In the proposed generation scheme,an external-cavity feedback semiconductor laser is utilized as a source of chaotic laser.Through quantizing the chaotic laser with 6-bit ADC, which is triggered by a clock at a sampling rate of 7 GHz,a binary sequence with six significant bits can be achieved. After the selection of the two least-significant bits and self-delayed exclusive-OR operation in the field-programmable gate array (FPGA),a real-time 14-Gb/s binary stream is finally achieved.This binary stream has good uniformity and independence,and has passed the industry-standard statistical test suite provided by the National Institute of Standards and Technology (NIST),showing a good statistical randomness.It is believed that this work provides an alternative method of generating the real-time high-speed random numbers and promotes its applications in the field of information security.