Quantum Light Meets Einstein to Prove Exactly Where You Are

Knowing where someone is has always seemed like a straightforward matter. You look, you measure, you trust your instruments. But in a world where communication, finance, and critical infrastructure increasingly depend on distributed networks of devices that may lie about themselves, the question of verifying a remote object’s position without trusting the object itself has become one of the most consequential open problems in information security. Now, a team of researchers reporting in Nature Physics has introduced and experimentally demonstrated a protocol that fuses two of the deepest pillars of modern physics, quantum optics and Einstein’s relativity, to accomplish exactly that: the remote verification of position using coherent states of light.

The core difficulty with position verification is that ordinary distance measurements are fundamentally based on trust. Radar, GPS, and time-of-flight ranging all rely on the assumption that the device being located responds honestly and that the signals it returns are genuine. A spoofing attacker can exploit this by relaying messages, predicting challenge responses, or replaying recorded signals faster than physics should allow. Classical cryptography alone cannot close these loopholes, because any classical challenge-response scheme can, in principle, be simulated or forwarded by a sufficiently capable adversary. What the new work shows is that by encoding challenges in quantum states, specifically coherent states, the kind of light produced by ordinary lasers, and by enforcing the relativistic speed limit on information, a verifier can confirm a prover’s location with security guarantees that no classical protocol can match.

The protocol builds on a concept known as relativistic position verification, which has been discussed theoretically for more than a decade. The idea is elegant in its use of special relativity. A verifier sends a challenge signal to the claimed position of a prover, and the prover must respond within a strict time window determined by the speed of light. If the prover is genuinely at the claimed position, the round-trip timing works out precisely. If the prover is anywhere else, the finite speed of light makes it impossible to receive the challenge and return a correct response in time. The catch, historically, has been that a single relativistic check can be defeated by multiple colluding adversaries who surround the claimed position and share information, provided the challenge itself carries no quantum advantage. The new protocol confronts this collusion problem directly by making the challenge a quantum state that cannot be perfectly copied or measured without disturbance.

Coherent states occupy a special place in quantum optics. They are the closest quantum states to classical light, describing the output of an ideal laser, and they are famously robust: a beam splitter tapping a fraction of a coherent state leaves the remaining light in another coherent state, undisturbed. This resilience is precisely why coherent states are the workhorses of optical communication. But it also means they cannot be protected by the no-cloning theorem in the same dramatic way that single photons can. The researchers’ insight was to design a verification scheme in which the security does not depend on detecting eavesdropping through disturbance, but rather on the statistical structure of the coherent-state challenge combined with relativistic timing constraints. An attacker who tries to intercept, measure, and forward the challenge gains only limited information within the light-speed-bounded window, and that limitation translates into a quantifiable, provable bound on the probability of successful spoofing.

In the experimental demonstration, the team implemented the protocol using quantum optical equipment in a laboratory setting that emulated the geometry of a multi-verifier network. Verifiers at separated reference points prepared coherent-state challenges and sent them toward the claimed position of a prover. The prover, located at the intersection point defined by the overlapping signals, was required to perform a joint measurement on the arriving light and return a response that depended on the full quantum content of the challenges. The verifiers then checked both the correctness of the response and its arrival time against the relativistically mandated schedule. Only a prover genuinely at the claimed spacetime point, with access to the complete quantum information carried by the coherent pulses, could satisfy both conditions simultaneously with high probability.

The measurement statistics from the experiment confirmed the central theoretical prediction. Honest provers at the correct location passed the verification test with high probability, while simulated attacks, in which adversaries positioned away from the claimed point attempted to collaborate and cheat the timing and content checks, succeeded only with probabilities bounded well below the levels required to break the protocol. The experiment thereby elevated relativistic position verification from a theoretical proposal with idealized assumptions to a demonstrated capability built from realistic optical components. Because coherent states are exactly what standard telecommunications lasers produce, the result carries an unusually direct path toward practical deployment in fiber networks and free-space optical links.

The security implications extend across several domains of modern technology. In satellite navigation, position verification could harden global positioning systems against spoofing attacks, a threat that has been demonstrated against civilian GPS receivers and poses risks to aviation, maritime shipping, and autonomous vehicles. In distributed computing and blockchain systems, verifiable position could anchor the physical identity of nodes, preventing adversaries from masquerading as geographically distributed participants. In quantum networks, where future quantum internet nodes will exchange entanglement and secret keys over continental distances, confirming that a node is physically where it claims to be adds a layer of authentication that no certificate or cryptographic key alone can provide. The protocol essentially gives the physical layer of communication a cryptographic guarantee derived from the laws of nature rather than from computational assumptions.

What makes the achievement scientifically notable is the way it reconciles two apparently competing demands. Quantum protocols for position verification have often relied on fragile single-photon states or entanglement, which are difficult to distribute over long distances and easily degraded by loss. Relativistic schemes, conversely, have been robust in their signals but vulnerable to collusion attacks. By choosing coherent states, the experimenters selected the most loss-tolerant, telecom-compatible quantum states available, and then recovered security against collusion through a careful protocol design that exploits the interplay between the quantum statistics of the light and the strict causal structure imposed by relativity. The result is a scheme whose ingredients are mundane, laser light and precise clocks, but whose guarantees are anything but.

The work also contributes to a broader conceptual shift in quantum information science: the recognition that spacetime structure itself is a computational and cryptographic resource. Just as entanglement enables tasks impossible classically, the light cone structure of relativistic spacetime constrains what any attacker can know and when they can know it, and protocols that weave quantum states through this causal fabric inherit guarantees rooted in physics. Position verification is perhaps the most natural application of this principle, because position is defined by spacetime, but researchers have begun exploring related ideas in secure timing, delegated quantum computation, and verifiable quantum communication. The new demonstration provides an experimental anchor for this emerging field, showing that the theory can be reduced to working hardware.

Challenges remain before such systems secure real-world infrastructure. Laboratory demonstrations operate over short distances with controlled timing, whereas field deployment will demand picosecond-level clock synchronization across verifier stations, management of atmospheric and fiber-induced noise, and careful analysis of loss, which affects coherent states in ways that must be folded into the security proofs. Scaling the number of verifiers and the distance to the prover will require engineering advances in optical timing distribution and high-speed quantum-light detection. Yet the direction is clear. The experiment demonstrates that verifying where someone is, without trusting anything they say, can be achieved by combining the most ordinary light in the universe with the most fundamental speed limit in nature. In doing so, it turns a century of physics, from Einstein’s postulates to the quantum theory of light, into a practical answer to a deceptively simple question: can you prove where you are? The answer, it turns out, is yes, if your answer travels at the speed of light and carries the quiet statistical fingerprint of a coherent state.

Subject of Research: Relativistic position verification using coherent quantum states of light

Article Title: Relativistic position verification with coherent states

Article References: Fan-Yuan, G.-J., Shan, Y.-G., Zhang, C., Wang, Y.-L., Fan, Y.-X., Xie, W.-X., He, D.-Y., Wang, S., Yin, Z.-Q., Chen, W., Fu, S.-N., Guo, G.-C., & Han, Z.-F. (2026). Relativistic position verification with coherent states. Nature Physics. https://doi.org/10.1038/s41567-026-03439-5

Image Credits: AI Generated

DOI: 10.1038/s41567-026-03439-5

Keywords: position verification, coherent states, quantum optics, relativistic cryptography, spoofing attacks, speed of light, quantum communication, laser light, quantum networks, spacetime, time-of-flight, security protocols