Experimental Demonstration of a Physical Zero-Knowledge Protocol for Nuclear Warhead Verification

Zero-knowledge protocols are a class of interactive proof systems that yield nothing beyond the validity of the assertion being proved. Developed for computational cryptographic appli- cations, their first non-trivial physical application was proposed by Glaser, Barak, and Goldston for the authentication of nuclear warheads. Here we report on the advancement of the experimental proof of concept of our physical zero-knowledge protocol for nuclear warhead verification. We present the first experimental demonstration of a zero-knowledge differential neutron radiographic protocol. The reference item (template warhead) and inspected items (treaty accountable items) are represented by objects of different fast neutron opacities made of steel and aluminum cubes. We illuminate items pre- sented for inspection using a D-T14 MeV neutron generator, and record their radiographs on identical sets of super-heated emulsion (bubble) detectors, specially developed for our application and previ- ously preloaded with the complement of the transmission image of the reference item. We find that, even with a modest number of bubbles, it is possible to discriminate between materials of different opacities and that zero-knowledge is leaked for inspected items identical to the reference item.