Checking whether a dismantled nuclear weapon actually contains a
real warhead is a tough job for weapons inspectors who aren't
permitted to collect classified information. But a novel technique developed by physicists could make nuclear
disarmament treaties much more effective.
"The verification of nuclear warheads for arms control involves
a paradox," explain the authors of a paper detailing the technique,
published in Nature. That paradox is that the
inspectors need to verify that a true warhead is present without
collecting any information about the materials or design of that
Traditionally, that's meant that arms negotiations have focused
on the delivery systems for nuclear weapons, rather than the
warheads themselves. It's somewhat effective to junk bombers,
submarines and ballistic missiles when they're the only way a
nuclear warhead will be used. But that's increasingly not the case.
Tactical and non-deployed nuclear weapons that aren't mounted on
long-range delivery systems are far harder to track.
"What we really want to do is count warheads," said physicist
Alexander Glaser, first author of the paper and an assistant
professor in Princeton's Woodrow Wilson School of Public and
International Affairs and the Department of Mechanical and
His proposed solution is designed in such a way that sensitive
information is never measured and so doesn't have to be hidden. A
warhead to be inspected is bombarded with high-energy neutrons, and
the number that escape to a detector is recorded. The same thing is
then done with a known real warhead.
But this would still record too much information, so the two
detectors are preloaded by the owner of the warhead to be tested
with a hidden number of neutrons. Crucially, the inspector then
decides which detector to use with which warhead. If the totals
differ, the submitted warhead would be proved as false. If they're
the same after several repetitions of the test, the warhead is
Glaser says he was inspired by computer science, and
particularly how passwords are verified online -- a technique known
as "zero-knowledge proof". He worked with Robert Goldston, a fusion
researcher at Princeton, and Boaz Barak, a senior researcher at
Microsoft New England who had taught computer science at Princeton
and is an expert in cryptology.
"This approach really is very interesting and elegant. The main
question is whether it can be implemented in practice,"
said Steve Fetter, a professor in the School of Public Policy
at the University of Maryland and former White House official. If
it can be, it could allow the more than 4,000 nondeployed strategic
and tactical nuclear weapons in US and Russian arsenals to be
included in the New Strategic Arms Reduction Treaty talks, which
currently call for each country to reduce its arsenal of deployed
strategic nuclear arms to 1,550 weapons.
It may also be useful in future negotiations to foster trust
between both sides. The authors of the paper note that any attempt
to bring the number of nuclear weapons in the world eventually to
zero "will likely require verification of individual warheads,
rather than whole delivery systems".