Daniel Gottesman and Emanuel Knill independently prove that a certain subclass of quantum computations can be efficiently emulated with classical resources.[13] 1999 Samuel L. Braunstein and collaborators showed that there was no mixed state quantum entanglement in any bulk NMR experiment. Pure state quantum entanglement is necessary for any quantum computational speedup, and thus this gave evidence that NMR computers would not yield benefit over classical computer. It was still an open question as to whether mixed state entanglement is necessary for quantum computational speedup[14] Gabriel Aeppli, Thomas Felix Rosenbaum and colleagues demonstrated experimentally the basic concepts of quantum annealing in a condensed matter system.