On the impossibility of extracting classical randomness using a quantum   computer
  In this work we initiate the question of whether quantum devices can provide us with an almost perfect source of classical randomness, and more generally, suffice for classical cryptographic tasks, such as encryption. Indeed, it is well known that classical computers are insufficient for either one of these tasks when all they have access to is a realistic imperfect source of randomness. On the other hand, quantum physics is inherently probabilistic which suggests that perhaps quantum computers can provide a reasonable way to overcome the above mentioned impossibility results. However, we show that this is not the case in a realistic setting where observations (measurements) are subject to noise. 
Yevgeniy Dodis and Renato Renner