Recent protests against the exclusion of Chinese scientists from an international conference held in the United States only scratch the surface of the formal and informal system of controls now in place to limit the access of foreign visitors to knowledge production at US research sites. American officials have enforced these controls with increasing rigor since 9/11 in response to charges of illicit data acquisition, notably by Chinese nationals. The "deemed export" rule is a particularly controversial instrument that officials have invoked to the detriment of scientific openness. It chips away at the freedom to do basic and applied unclassified research with foreign students, imposing tight restrictions on mundane research practices of knowledge sharing. An analysis of official documents, as well as interviews with research engineers and managers at the Georgia Institute of Technology, lays bare the concerns in academia over the impact of these tighter rules on the educational enterprise. It is impossible to stop all leaks of sensitive information abroad. Security lies in selectively building high walls around particularly sensitive knowledge in a climate of openness.