Made from the Right Stuff
Most people are familiar with the reddish glow of a neon sign, the orange of a sodium street lamp, or violet of a mercury black light. The colors of these lights are signatures of the elements emitting the light, each color unique to each element and determined by the energies of the electrons around that element's nucleus. In an analogous way, the protons and neutrons inside the nucleus are associated with another set of energies, and these too can glow with a unique set of colors, except the light from the nucleus is x-rays instead of visible light. By observing the spectrum of these x-rays, we can determine not only the elements, but also isotopes from which an object is made. This is called Nuclear Resonance Fluorescence (NRF), and it lets us determine the exact composition of the nuclear warhead. Unlike other nucler measurements, it is impossible to substitute one isotope or element for another.
All In the Right Place
Not only do we need to check all the materials used to make the warhead, it's also necessary to determine that the warhead has all those materials in all the correct locations. By measuring the NRF signature of the test object from many different angles, we in effect create a tomographic map of the distribution of isotopes within the object.
Keeping Secrets Secret
The challenge of warhead verification comes in protecting secret information. A straightforward measurement of the warhead's composition and geometry would reveal far too much information to the inspector. One possibility long pursued is to use software or electronics to analyze the measured data and hide it from inspectors, giving only a pass/fail result. Unfortunately, nobody has yet figured out how to make that approch perfectly secure against hacking or cheating. Instead, we use use a physical instantiation of one-time-pad encryption to protect information. The process is based on a special single-pixel tomographic transform and the use of a physical secret key for each single-pixel measurement. The physical secret key is simply a thin foil, the composition of which is known only to the weapon owner. The x-rays leaving the weapon are convolved with the foil-key. The result is the equivalent to an enrypted bit representing the line-integrated density of one isotope in the warhead. By taking multiple projections, corresponding to multiple lines of interrogation through the warhead, the equivalent of a digital hash is built up. By comparing the hash values for a real and test warhead, one can confirm if the test warhead is a match or not.