A newly developed system can check the presence and number of nuclear weapons in a secret warehouse without sharing information about the design or content of the weapons.
Nuclear weapons are one of the greatest threats to humanity. A nuclear war could cost millions of lives and cause irreparable damage to the environment. That is why it is important that there are international treaties that limit and control the possession, proliferation and use of nuclear weapons. But how do you know whether a country adheres to these treaties? How can you check whether a country secretly has more nuclear weapons than it claims? And how can you do that without violating the national security or privacy of that country?
“About seventy percent of the world’s nuclear weapons are held in military reserve or awaiting dismantling,” says Sebastien Philippe. “It is difficult to see exactly how many there are via satellite images or other methods that do not allow you to look into the storage vaults.” Because the exact number is difficult to verify, many nuclear weapons are not included under existing nuclear disarmament treaties. To change this, an international team of researchers has developed a new technique: a system that uses radio frequencies to determine the presence and number of nuclear weapons in a storage facility.
Send back and forth
The system works as follows: an inspector (state A) sends a radio signal to a storage facility (state B) where he wants to know whether there are nuclear weapons. The radio signal is reflected by the walls, ceiling, floor and any objects in the room, including nuclear weapons. The reflected signal is captured by two antennas that create a kind of ‘radio fingerprint’ of space. This radio fingerprint is encrypted using a mathematical function and sent back to State A. State A can then compare this radio fingerprint with a fingerprint created earlier, when there were no nuclear weapons in space. If there is a difference between the two fingerprints, it means that something has changed in space, for example, nuclear weapons have been added. State A can then estimate how many nuclear weapons there are by looking at the size and shape of the difference.
Mirrors
In theory, the opponent to be controlled could bypass the signals, the scientists reasoned. This can be done by actively sending back a kind of recording of the room, instead of the measured signal. “That would be like someone sticking a photo in front of a surveillance camera,” says lead author Johannes Tobisch.” To prevent this, the radio signal is not sent directly, but via a series of mirrors that are controlled randomly, with each mirror slightly distorting the radio signal. Because the transmitter knows how each mirror is set, this party can reconstruct the returned radio signal.
The recipient can only guess how each mirror is set up, and will therefore not have enough time to send back fake information as a circumvention. “For example, if you used twenty mirrors to distort the signal, it would take about eight weeks to decipher the underlying mathematical function,” Tobisch explains. “The more mirrors you add, the more difficult it becomes for the opponent to decipher, and the more secure the method is.”
To test the technique, the team simulated nuclear weapons using metal cylinders in a laboratory. This showed that the system works and that it does not leak information about the design or the precise contents of the weapons. In this way, the country’s privacy is not damaged, “which makes new levels of trust possible,” the researchers said. The hope is that the new verification technology can contribute to treaties that aim to limit all types of nuclear weapons. “At a time of heightened geopolitical tensions and with a new nuclear arms race in the making, this work is particularly timely and relevant,” concludes Philippe.
