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One of the fundamental tenets of quantum mechanics is that measuring a physical system always disturbs it. If the system in question is a message in a series of digital bits encoded in the polarisation of light, this means that intercepting and reading the message can no longer be done surreptitiously. The receiver should be able to detect an eavesdropper and take appropriate countermeasures.

In practice, quantum-key-distribution systems rely on sophisticated optical equipment to prepare, transmit and detect the individual polarised photons that make up the key. And when these real-world components meet the clever academic theorems that guarantee security, holes emerge.

Research conducted by the Norwegian University of Science and Technology, the National University of Singapore and the University of Toronto hacked into a system that connects several buildings on the National University of Singapore’s campus. For the first hack, small eavesdropping apparatus was designed to take advantage of a weakness in a particular kind of photon detector in the receiver’s receiving equipment. The second hack was carried out by a team from the University of Toronto, led by Professor Hoi-Kwong Lo (ECE), which stole information from a research version of a system made by ID Quantique—a Swiss firm trying to commercialise quantum cryptography—by taking advantage of synchronisation signals that pass between the sender and receiver.

As quantum hackers continue to put systems through their paces, such loopholes will be closed—as these now have been—and the systems become more secure.

Follow the link to read the full article in The Economist.

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