  Future nuclear arms reduction efforts will require technologies to verify
that warheads slated for dismantlement are authentic without revealing any
sensitive weapons design information to international inspectors. Despite
several decades of research, no technology has met these requirements
simultaneously. Recent work by Kemp et al. [Kemp RS, Danagoulian A, Macdonald
RR, Vavrek JR (2016) Proc Natl Acad Sci USA 113:8618--8623] has produced a
novel physical cryptographic verification protocol that approaches this treaty
verification problem by exploiting the isotope-specific nature of nuclear
resonance fluorescence (NRF) measurements to verify the authenticity of a
warhead. To protect sensitive information, the NRF signal from the warhead is
convolved with that of an encryption foil that contains key warhead isotopes in
amounts unknown to the inspector. The convolved spectrum from a candidate
warhead is statistically compared against that from an authenticated template
warhead to determine whether the candidate itself is authentic. Here we report
on recent proof-of-concept warhead verification experiments conducted at the
Massachusetts Institute of Technology. Using high-purity germanium (HPGe)
detectors, we measured NRF spectra from the interrogation of proxy 'genuine'
and 'hoax' objects by a 2.52 MeV endpoint bremsstrahlung beam. The observed
differences in NRF intensities near 2.2 MeV indicate that the physical
cryptographic protocol can distinguish between proxy genuine and hoax objects
with high confidence in realistic measurement times.
