Quantum physics can confirm someone’s whereabouts

DENVER — An urgent call comes in from the White House. But the recipient is skeptical: he needs a way to verify that the message comes from the presumed location. Quantum physics has a solution.

Scientists have demonstrated a quantum technique for make sure someone is where they say they arephysicist Abigail Gookin reported March 18 at the American Physical Society’s World Physics Summit. Called quantum position verification, the technique is based on the concept of quantum entanglementin which the fates of two distant particles are closely linked.

In the future, this technique could be useful to prevent certain types of phishing attacks or to limit user access to certain resources. (For example, access to sensitive infrastructure related to nuclear weapons could be limited to people in a secure government building.) The method could be part of a future as much as the Internet which could one day provide different types of ultra-secure communications.

Here’s how it works: two people, called verifiers, each want to confirm that a third person, called a prover, is at a given location. The verifiers, located on opposite sides of the prover’s presumed location, each send a random number to the prover, which the prover will use to determine its next step. Meanwhile, one of the verifiers creates a pair of entangled photons, or particles of light. The verifier keeps one photon and sends the other to the prover.

The prover and verifier measure their photons simultaneously. Specifically, they measure polarization, the direction in which electromagnetic waves from photons wiggle. The random numbers tell the prover which measurement parameters to use to determine the polarization of their photon. The prover returns the result of its measurement to the verifiers.

Then the verifiers compare the prover’s result to the measurement result of the other photon. Over many repetitions of this photon measurement protocol, the results should be strongly linked or correlated. If a sneaky impostor intercepted the photon from another location, the measurement results would not be as strongly correlated as expected, due to constraints imposed by the speed of light and the quirks of quantum measurements. The auditors would know something was wrong.

At the National Institute of Standards and Technology (NIST) in Boulder, Colorado, scientists created two verification stations, separated by about 200 meters and connected by optical fibers to an intermediate standard. The successful method located the proverNIST’s Gookin and colleagues reported in a paper submitted Jan. 23 to arXiv.org.

The basis of this technique is what is called a flawless Bell test, a method of sending and measuring quantum particles that proves that the quirks of quantum physics cannot be explained. Indeed, the correlations between pairs of photons are stronger than any classical theory could explain.

Because this technique connects an actual physical location to information exchanged by parties, it could serve as the basis for other secure communication techniques, Gookin says. “This is the first time we can concretely link a person’s location to their information.”