Most quantum networks send secret data in the polarization of photons. The sender encodes each photon’s polarization such that the receiver who tries to measure it will only get the right information out about half of the time. When this information does come through, the duo can agree to use that particular bit of data as a key to encode and decode a message.
The system ensures secrecy because anyone intercepting a transmitted photon will disrupt its polarization, and affect the rate at which the receiver can correctly measure it. So the sender and receiver can detect the eavesdropper by noticing a spike in the transmission error rate. They can then stop communicating or try again on a different network.
Shapiro and his co-authors have successfully executed a trick that gets at least part-way around this. To listen in, the team used a quantum-mechanical principle known as entanglement, which can link together two different traits of a particle. Using an optical setup, the team was able to entangle the transmitted photon’s polarization with its momentum. The eavesdropper could then measure the momentum in order to get information about the polarization, without affecting the original polarization.