Treaty Verification with Resonant Phenomena
Nations have been working to reduce the size of Cold War nuclear arsenals for more than thirty-five years. Because parity in force capability remains a central factor in strategic stability, reductions are possible only with intricately negotiated verification provisions that ensure neither side is cheating. Historically, verification was limited to delivery vehicles such as missiles and aircraft; it has not included checks on nuclear warheads directly. The limited approach allowed meaningful reductions in the level of deployed nuclear forces while avoiding the difficult challenge of having foreign governments inspect highly classified nuclear warheads. Ultimately, however, direct verification of warheads will be needed to facilitate warhead dismantlement, to eliminate the possibility of rapid rearmament, and to reduce the risk that “loose nukes” for the bloated US and Russian arsenals fall into the hands of terrorists. While many of the barriers to ambition, warhead-focused and verification-driven treaties are political, it is also clear that new technological research is also necessary to help solve this problem.
Despite decades of work, no protocol for direct warhead verification has been developed that is able simultaneously to provide high assurance that a purported warhead is real and protect the secrets of a warhead's design. The currently favored approach is to measure specific parameters about the weapons, then a software-driven "information barrier" processes the data to determine if they are within parameters, giving a sanitized pass/fail determination. There are several problems with this approach. One is that the pass conditions must be loosely defined so that the conditions will not themselve suggest information about the warhead design. As a result, the test is not precise and could erroneously authenticate non-weapon objects. A second problem is that the information barrier may itself provide an opportunity for cheating. Electronic systems are vulerable to software hacks and side-channel exploits. As yet, there is no established method for authenticating the invulnerability of electronic information barriers.
Can hack software. Can't hack physics.
We have proposed a system that has the potential to measure a warhead more precisely and protect the measured data not with an information barrier, but with physical cryptography. In physical cryptography the weapon-specific information is modified in a predictable but unknown way through the physics of the measurement process. This occurs before an electronic detector can record the data. Participants do not need to trust each other's devices; they only need to trust their understanding of physics.
There may be several ways in which the physics might be exploited to effectively encrypt a measurement, but the masurement must also be a meaningful test of the warhead's authenticity. We defined the goal of testing that all coponents of a warhead are made of the correct material, down to the isotopic mix; have the correct geometry; and the correct relative placement inside the warhead. Simple radiography (with neutrons or photons) will capture spatial information, but tend to be isotope- and even element-agnostic. To achieve isotopic sensitivity, we propose to measure resonant interactions with nuclei, which are sensitive to isotopes.
Most bound states--a guitar string, a wine glass, the electrons of an atom, or the nucleons of a nucleus--have a very selective way in which they couple to external excitations. The guitar tuner, for example, emits sound of varying frequency, and when that frequency matches that of the string it resonates. Your tuner knows if you hit A2, and not E3, because the resonance is unique; it will never mistake an E for an A.
The atoms and nuclei--states of electrically bound electrons, or nucleons bound by the strong force--are the quantum equivalents of the guitar string. When impinged upon by external particles (photons, neutrons, electrons) they strongly interact if those particles carry energy resonant with the transitions from the ground state to some excited level. This interaction may cause the nucleus to absorb the particle and enter into an excited state. After a short while, the nucleus can de-excite, emitting a secondary particle. The emitted or absorbed particle's energy help us to identify the isotopic identity of a nucleus, just as the guitar tuner can identify the note of each string.
We are studying two types of resonant processes for verification:
Nuclear Resonance Fluorescence (NRF)
NRF involves the interaction of ~MeV photons with nuclei, causing very energy-specific interactions in most isotopes. By measuring these, one can achieve a physical cryptographic warhead verification. Learn more about NRF-based Physical Cryptographic Verification
Epithermal Zero Knowledge (epiZK)
Epithermal neutrons in the ~eV range exhibit sharp resonant interactions with most actinides. This behavior can be used to perform highly isotope-sensitive warhead component verification. Learn how we achieve Verification with Epithermal Beams.