Science1 publisher2 min readPublished
A USTC team pins a device's position to a 74.3-meter window using verifiers 2 km apart
A group at the University of Science and Technology of China combined quantum measurement with relativity's speed limit to test where a device claimed to be, and pinned it to a 74.3-meter stretch between two stations.
The Scientist · Science desk
What happened
- A team at the University of Science and Technology of China, writing in Nature Physics, verified a device's claimed position using two verifiers 2 km apart and narrowed the possible location to 74.3 meters.
- In the protocol, one verifier sends a quantum state while both send classical information from different directions, and the prover must use that information to choose its measurement and answer at once.
- The methods in use today can be fooled by counterfeit satellite-navigation signals, edited software coordinates, or relay attacks that intercept and forward verification messages.
- The group came to the problem from years of quantum key distribution work, asking what a secure connection between two parties can be used for beyond secure communication.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint Every pair of verifiers needs a secure connection to each other before a single position claim can be tested, so where this works is bounded by where that infrastructure already runs.
- capability A verifier that gets a correct answer within the timing window learns something about location without trusting the coordinate the client reports or the software reporting it.
- decision A 74.3-meter window sorts the plausible applications from the implausible ones: it can distinguish a facility from the streets around it, and cannot distinguish one floor from the next.
74.3 meters is how far light travels in about 248 nanoseconds [14]. That sets the scale of the timing uncertainty the whole chain has to hold: the photon source, the detectors, the classical links coming in from both sides, and the prover's own response [4]. Against the baseline, the window is 3.7 percent of the 2 km between the two verifiers [13].
The verifiers sit on opposite sides of the point being checked [4]. Two stations define a line, and the region the experiment leaves open is a 74.3-meter stretch of it [17].
Neither half of the challenge is useful on its own. "In this sense, the quantum state carries the information to be measured, while the classical information remotely determines how it should be measured," Guan-Jie Fan-Yuan, Shuang Wang and Zhen-Qiang Yin told Phys.org [5][9]. An attacker standing somewhere else would have to hold an unknown quantum state until the classical instructions reached it, or pass information between the two sides faster than light. "The security comes from combining two fundamental principles. Quantum mechanics places fundamental limits on an adversary's ability to copy or extract information from an unknown quantum state, while relativity limits how quickly spatially separated adversaries can exchange information," they said [6].
The protocol assumes the verifiers already trust each other. Fan-Yuan, Wang and Yin described the approach as "allowing multiple distributed verifiers with established secure connections to verify the claimed position of a remote prover, with security rooted in fundamental physics" [11].
The phys.org report gives the 2 km baseline and the 74.3-meter window. It does not report the loss, error rates or latency the system achieved, or any adversary trying to beat the protocol [16]. Loss, error rates and latency decide whether this scales past a 2 km testbed, and the authors put them at the center of the difficulty: "In practice, quantum signals inevitably suffer from loss and errors, while position verification also places extremely demanding requirements on system latency," they said [7].
The uses they name are authorizing financial transactions, securing communications, and controlling who can access specific databases or services [12]. Each of those is a check on a claim. The authors draw the line between this and satellite positioning there: "the challenge is not simply to determine a position, but to make that position trustworthy," they said [8].
What to watch
- Whether the Nature Physics paper publishes loss, error-rate and latency figures, and a stated adversary model, behind the 74.3-meter claim.
- A demonstration with three or more verifiers. That would take the result from a line segment to an area.
- Any run over a metropolitan link of tens of kilometers, where both channel loss and latency grow.