Nuclear disarmament is all about trust—a hard thing for rival political powers to build, even under the best of circumstances. Today, a team of researchers revealed something that might make that process easier: a new technique that nuclear inspectors can use to verify whether a warhead is active, inactive, or a fake—all without learning anything about its design. "This is something that has been an open problem for 50 years,” says study author and Massachusetts Institute of Technology in Cambridge nuclear policy expert R. Scott Kemp, who adds that it may be “the first technical solution” to the problem.

Nuclear powers still possess more than 15,000 such weapons, according to the Federation of American Scientists. Although agreements such as 2010's New Strategic Arms Reduction Treaty have sought to limit U.S. and Russian stockpiles of nuclear missiles, no system has been successfully implemented to verify whether a nuclear warhead has been dismantled without revealing design secrets. Previous methods that kept those secrets safe were all vulnerable to cheating, Kemp says.

The new method—still theoretical—is a kind of physical encryption that allows warhead scanning without revealing too much. Inspectors would send a high-energy x-ray beam through a warhead in which some of the x-ray photons would be absorbed by nuclear materials. As in medical imaging, the pattern of x-rays that emerges can reveal the geometry of what’s inside the warhead. To scramble that information, the emerging beam is directed onto an "encryption foil"—a set of materials picked by the warhead's owner and not revealed to the inspector. The x-ray photons hitting the foil cause it to emit gamma rays, via a process known as nuclear resonance fluorescence, which can then be picked up by a nearby detector.

The amount and frequencies of the gamma rays would provide a signature of the layout and composition of the warhead. But, because of encryption foil, that layout and composition would not be discernible to the inspectors. If the inspectors can first test a known active warhead and use it as a model, they would then be able to identify the status of any subsequently tested warheads with 99.9% accuracy, the researchers write today in the Proceedings of the National Academy of Sciences . Because the identity of the foil's material will remain unknown to the inspectors, it would be impossible for them to use the detected radiation to infer the design of the warhead.

Earlier methods to achieve such verification were still open to deception, the researchers say. Plasma physicist Robert Goldston of Princeton University, who helped develop one such system using high-energy neutrons, says he found Kemp's system very interesting. He hopes that more work will be done to develop the technology to implement it successfully, perhaps in parallel with his neutron-imaging system. However, any real-world application of so-called “zero-knowledge” warhead verification remains decades away, Kemp says, thanks to—what else—diplomacy.