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Lasers illuminate the quantum security gap

The result of a false ‘entanglement’ defies cryptographic technique.

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Quantum cryptography is supposed to be virtually impossible to crack, but hackers have repeatedly found ways to beat the system.
Credit: VOLKER STEGER/SPL

Hackers have cheated a gold standard test of quantum cryptography. By using lasers to help spoof the quantum property of entanglement, they have challenged attempts to build uncrackable cryptographic systems.

Quantum cryptography, which uses the quantum states of light particles called photons to encode information for transmission, takes advantage of the fact that you can’t make measurements of a quantum system without disturbing it. This means that, in principle, it is impossible for an intruder to intercept a quantum encryption key without breaking it and triggering alarms. In practice, however, technological weaknesses in the device provide opportunities for hackers. In 2010, two independent groups successfully cracked two commercial quantum cryptosystems and evaded detection.1,two.

“There have been some strong claims about the robustness of quantum cryptography against any attack,” says Christian Kurtsiefer, an expert in quantum optics at the Center for Quantum Technologies at the National University of Singapore. “But it’s not that simple”.

These breaches have prompted physicists to try to build more complex devices that can generate a quantum key (encoding classical bit values ​​of 0 and 1 in two different polarization states of photons) and incorporate a push-button test to certify that the key is still safe, says Kurtsiefer. This strategy uses a chain of entangled photons, light particles that twin in such a way that measuring the polarization state of one instantly modifies the polarization state of its partner. Two parties, ‘Alice’ and ‘Bob’, share a quantum key by each taking one member of each generated entangled pair.

Any attempt to eavesdrop by intercepting Alice’s or Bob’s photons will destroy the entanglement. To check whether this has happened, the system incorporates a standard entanglement test, known as a Bell test, which compares how well Alice’s and Bob’s photon polarizations match: if the particles are correlated above a certain threshold, then the entanglement is confirmed. and the key is certified as secure.

But Kurtsiefer and his colleagues managed to cheat on Bell’s test. In the test, photons from Alice and Bob are picked up by detectors that differentiate between polarization states representing 0 or 1. Kurtsiefer’s team ‘blinded’ Bob’s detector by shining a laser beam at it, and then intercepted their photons, reading its polarization values. While he is blinded, he can trick the detector into registering a value of ‘1’ every time the hacker fires an additional laser pulse at him. So when the researchers intercepted a real value of 1 in Bob’s photons, they fired a pulse at their blinded detector, leading Alice’s and Bob’s detectors to register false correlations, mimicking entanglement. The results appear in Physical Review Letters3.

An idealized Bell test should still have been able to identify them as spoofed correlations because the team’s signals couldn’t have perfectly matched Alice’s photons every time, Kurtsiefer notes. However, this was not noted because actual Bell tests in the lab allow for a certain amount of misalignment due to imperfections in the equipment. “Even today’s best photon detectors only pick up a fraction of the photons in the apparatus,” says Kurtsiefer.

This “detection loophole” allows hackers to disguise missing correlations as nothing more sinister than the expected inefficiency of the detector.

Antonio Acín, a quantum physicist at the Institute of Photonic Sciences in Barcelona, ​​Spain, admits that in the past, physicists have been tempted to ignore the detection loophole. “These devices are so technologically demanding that to make them more feasible to build, we sometimes think about sacrificing robustness,” he says. “This experiment shows that we shouldn’t do that.”

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Christian Kurtsiefer

Antonio Acin

About this article

Cite this article

Merali, Z. Lasers Illuminate Quantum Security Loophole.
Nature (2011). https://doi.org/10.1038/news.2011.611

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