Oak Ridge Physicists Patent a Quantum Mechanical Burglar Alarm

Quantum mechanics could find a near-immediate real-world application in an unexpected place: burglar alarms.

This is according to physicists at Tennessee's Oak Ridge National Laboratory who describe a "tamper indicating quantum seal" in the current issue of the journal Physical Review Applied (a paper which can also be found at arXiv). Imagined uses for the technology include safeguarding nuclear materials and sensitive documents.

In a sense, this is a natural enough utility of quantum mechanics. This is, after all, the gist of quantum encryption—observing a quantum system necessarily changes it, so if we send information along a channel in a particular quantum state, we'll know at the other end if the information has been intercepted because the state will be different than expected.

The Oak Ridge scheme extracts this idea beyond bits and into the world of physical materials.

High-tech tamper proofing mechanisms already depend on something a bit like a non-quantum (or classical) version of what I described above. Basically, fiber-optic cabling is wound around whatever it is that needs to be safeguarded and a continuous signal is sent through it. If the cabling is disturbed, its properties will be altered and the signal will change. This is a common enough system and has even been suggested for human-monitoring bracelets.

Fiber-optic intrusion detection systems can be beaten, however. The hack sounds simple enough: the eavesdropper needs only to observe the incoming signal and then reproduce it on the other side of the intrusion. The detector then only sees the undisturbed signal. This is called "spoofing" or an "intercept-resend" attack.

"In the classical setting, detection of tampering requires failure of the intruder to accurately replicate the original transmission," the Oak Ridge group, led by quantum physicist Brian P Williams, writes. "This is typically accomplished with 'secret' information that is hidden from the intruder, for example, the optical modulation sequence used to transmit pulses. This secret information, however, is vulnerable to discovery by the intruder using conventional signal detection methods. Thus, in principle, an attacker is able to perfectly replicate the optical signal using a priori knowledge, and the classical variant of an optical seal is vulnerable to an intercept-resend spoofing attack."

The Oak Ridge detection scheme depends on entangled pairs of photons (light particles). The basic idea of entanglement is that two seemingly individual particles are allowed to share a single quantum state, even as they become separated over potentially unlimited distances. The effect is almost as if a single particle is allowed to exist in more than one place at a time.

One implication of quantum entanglement or quantum systems, generally, involves measurement. If someone disturbs one of the particles in a quantum state, this will have an effect on the other particle(s), not matter how far away the other particle(s) might be. (Or, you can imagine someone disturbing one particle and the same effect being seen at the other manifestation of that particle, no matter how distant it might be.) It's pretty weird.

You can see where this is going. The Oak Ridge group's idea was to take a stream of entangled photons and split it up, entangled pair by entangled pair. One photon partner/mirror goes directly to a detector, while the other travels along the fiber-optic intrusion detection circuit and eventually rejoins its partner at the detector.

If no intrusion was detected along the circuit, the result of this will be pairs of entangled particles back at the detector. If particles were entangled such that they were linked via, say, the particle property of polarization, such that one had an "up" polarization and the other had a "down" polarization, and particle pairs started arriving at the detector with matching polarizations instead, something clearly would be amiss.

This works because it's not possible to spoof a quantum state. Once it's been measured, the information is just gone. The no-cloning theorem of quantum mechanics states that it's impossible to make a perfect copy of an unknown quantum state.

In experiments using a 20 meter optical fiber, the Oak Ridge group was able to detect tampering with a probability of 99.99 percent. Basically perfect.

As noted in an APS Physics summary of the work, Williams and co. have already patented the technology.

"The tamper-indicating quantum seal presented here offers unprecedented surety in the detection of intrusion against fiber optical seal systems," the Oak Ridge group concludes. "Applications in containment and surveillance technologies, as well as telecommunications security and fiber-based sensors, are likely to benefit from the adoption of these ideas."