At top, a diagram shows the configuration that could be used to verify that a nuclear warhead is real. At left, the key component of a nuclear weapon, called the pit, which consists of a hollow sphere of plutonium, is lined up with a specially made second component, called the reciprocal, which has the opposite characteristics. When the two are observed using a beam of neutrons, the resulting image (bottom left) is distinctive but reveals no details of the pit’s dimensions and composition. However, if an outwardly similar-looking object with a different shape or composition is substituted for the pit, it results in a distinctly different image, making it easy to tell that it’s a fake.
In past negotiations aimed at reducing the arsenals of the world's nuclear superpowers, chiefly the U.S. and Russia, a major sticking point has been the verification process: How do you prove that real bombs and nuclear devices - not just replicas - have been destroyed, without revealing closely held secrets about the design of those weapons? Now, researchers at MIT have come up with a clever solution, which in effect serves as a physics-based version of the cryptographic keys used in computer encryption systems. In fact, they've come up with two entirely different versions of such a system, to show that there may be a variety of options to choose from if any one is found to have drawbacks. Their findings are reported in two papers, one and the other in the Proceedings of the National Academy of Sciences , with MIT assistant professor of nuclear science and engineering Areg Danagoulian as senior author of both. Because of the difficulties in proving that a nuclear warhead is real and contains actual nuclear fuel (typically highly enriched plutonium), past treaties have instead focused on the much larger and harder-to-fake delivery systems: intercontinental ballistic missiles, cruise missiles, and bombers. Arms reduction treaties such as START, which reduced the number of delivery systems on each side by 80 percent in the 1990s, resulted in the destruction of hundreds of missiles and planes, including 365 huge B-52 bombers chopped into pieces by a giant guillotine-like device in the Arizona desert.
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