Focus: Burglar Alarm Based on Quantum Mechanics

Physics 9, 2
Researchers demonstrated a scheme that relies on quantum mechanics to prevent unauthorized access to restricted objects, such as nuclear materials.
Do not touch. A new quantum-mechanical protocol could allow improved detection of tampering with sensitive materials.

The detection of unauthorized tampering—in stores of anything from sensitive documents to nuclear materials—is improved by exploiting the basic properties of quantum physics, according to a new study. Researchers demonstrated that optical monitoring systems, if made sensitive to quantum properties of light, could avoid vulnerabilities afflicting the best current tamper-detection systems. The new technique could soon find its way into practical use.

Advanced systems for tamper protection currently rely on fiber-optic cables threaded through and around stores of materials to be safeguarded. These cables carry secret optical signals, which traverse the path and return to a detector at a secure site. Efforts to tamper with the materials, by disturbing the cables, would alter the returning signal and reveal the intrusion.

But such systems remain open to a conceptually simple attack. A sophisticated intruder, upon breaking the fiber, could hide his or her presence by reproducing, or “spoofing,” the original signal. They need only detect the arriving signal and send along an exact copy. Such an “intercept-resend” attack would produce precisely the signal expected, and so avoid detection.

This problem can be overcome by using optical signals that are far harder to spoof, argues a team of researchers from the Oak Ridge National Laboratory in Tennessee led by Travis Humble. Quantum physics implies that photons can be created as so-called entangled pairs, where a disturbance to a physical property of one particle—say, its frequency or polarization—could also immediately alter the same property in its partner particle. In calculations and experiments, the team showed that such pairs could be used to thwart intercept-resend-style attacks.

Their idea was to create a stream of entangled photon pairs. For each pair, one would be sent directly to a nearby detector in a secure place, while the other would instead travel down a fiber and through the protected zone, before joining its partner at the detector. In the absence of any disturbance along the way, the second photon, upon reaching the detector, would still show a perfect entanglement with the first. For example, the two photons might have been created in a state with opposite polarizations: if one is vertical, the other must be horizontal. In contrast, any disturbance to the fiber would alter properties of that second photon, destroying the entanglement, so that some pairs might have the same rather than opposite polarizations.

Moreover, even a spoofed imposter particle could not replicate the expected entanglement. That impossibility, says team member Brian Williams, follows from the “no-cloning” theorem of quantum theory, which rules out the possibility of making a perfect copy of an unknown quantum state. Measuring the intercepted photon would destroy any information about its original state.

To demonstrate the scheme, the researchers generated entangled photon pairs and sent each photon through a 20-meter fiber and then back to a detector. One loop was meant to simulate passage through the protected materials; the other acted as a reference. Within one loop, the team inserted a transparent but weakly disturbing obstacle in the photon path to mimic a tampering event. In the experiments, they were able to detect the presence of the obstacle with a probability of 0.9999, while also maintaining a false positive rate of only one part in one billion.

The new method, however, would be vulnerable to still more sophisticated attacks. The no-cloning theorem prevents an attacker from measuring and reproducing a photon's state, but quantum mechanics still allows for teleportation, where the unknown state is destroyed in the process of transferring it from one photon to another. However, Williams says, teleportation requires much more sophisticated equipment, especially given the picosecond timing that would be required to prevent the security system from noticing delayed photons.

“You will probably not want to invest in this beautiful quantum seal to protect the safe in your hotel room,” says Pepijn Prinkse of the University of Twente in the Netherlands. But he says there are many situations where the complexity is worthwhile, such as bank vaults or high security computer servers, in addition to nuclear materials.

Oak Ridge National Laboratory has now patented the system, and Williams and his colleagues hope that it will soon come into practical use. Williams says the system is ready for the rigors of the real world. "While quantum states are typically considered quite fragile, as quantum states and entanglement verification goes, ours is very robust."

This research is published in Physical Review Applied.

–Mark Buchanan

Mark Buchanan is a freelance science writer in Normandy, France.

Subject Areas

Quantum Information

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