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

For many years, experimental attempts to realize and harness Majorana bound states have primarily targeted extended one-dimensional semiconductor-superconductor hybrid structures. However, such systems have suffered from intrinsic disorder due to material imperfections and fabrication limits, severely hampering consistent, reproducible creation and manipulation of Majoranas. This disorder blurs the quantum coherence and complicates interpretation of experimental signatures. Senior researcher Srijit Goswami and his team at Delft University of Technology recognized this fundamental roadblock. “The material quality imposes a ceiling on how reliably we can engineer and probe Majorana quasiparticles in these traditional architectures,” Goswami remarks, highlighting the immense challenge faced by the community.

In a bold and innovative approach, the Delft team revisited the foundational Kitaev chain model, a simplified theoretical framework proposed in 2000 that predicts the existence of Majorana modes in a linear chain of coupled quantum sites. Rather than relying on naturally disordered materials, they constructed a highly controllable model system from the ground up, using chains of artificial atoms called quantum dots. These zero-dimensional nanostructures behave as tunable quantum “sites”, whose interactions can be engineered with exquisite precision. By linking three such quantum dots and connecting them via superconducting elements, the researchers recreated a minimal Kitaev chain, allowing them to probe the emergence and stability of Majorana bound states in a clean, deterministic platform.

This quantum dot chain approach offers an elegant platform to systematically explore how Majorana modes evolve and interact. Prior efforts at QuTech had investigated shorter, two-site versions of Kitaev chains in various materials, including semiconductor nanowires and two-dimensional electron gases (2DEGs). Building on these efforts, the current work successfully extends these chains to three quantum dots, opening the door to richer phenomena and improved control. Crucially, the architecture employs 2DEGs combined with superconducting Aluminum strips, enabling both the formation of quantum dots through gate voltages and the hybridization needed to induce superconductivity. First author Bas ten Haaf emphasizes the significance: “By fine-tuning the coupling between these quantum dots, we observed Majorana bound states appearing simultaneously at opposite ends, while the central dot mediates the bulk properties in perfect agreement with Kitaev’s predictions.”

Central to the topological protection in these systems is the concept of the “bulk gap,” an energetic separation in the middle of the chain that isolates the edge Majoranas and prevents their mutual annihilation. Remarkably, this bulk gap is tunable in the Delft setup, controlled by the middle quantum dot’s parameters. When the researchers removed this gap by adjusting gate voltages, the spatially-separated Majorana bound states on the ends lost their stability and merged—an elegant demonstration of Kitaev’s toy model in real semiconductor-superconductor circuits. This tunability not only validates theoretical models but also offers experimental knobs to manipulate and understand Majorana physics in an unprecedented way.

While numerous experiments in the past have reported signals consistent with Majorana modes, this work is distinctive in its ability to simultaneously probe left, center, and right sections of the tri-dot chain. Such spatially resolved measurements provide a clearer, more comprehensive picture of the quantum states involved and allow unambiguous identification of topological signatures. This degree of control and resolution marks a significant milestone, enhancing the reliability of Majorana detection. Moreover, the experiment’s minimalistic approach removes many complicating factors inherent in extended nanowires, thereby isolating the fundamental physics underpinning these exotic states.

Beyond observation, the Delft researchers demonstrated the dynamic control of Majorana bound states’ locations within the chain. By modulating the couplings between neighboring quantum dots, they could effectively move the Majoranas from one site to another. This capability to shuttle Majoranas spatially is not merely a technical tour de force—it is an essential requirement for the realization of topological quantum computing. The theoretical robustness of information encoded in Majoranas hinges on performing “braiding” operations, where exchanging positions of Majoranas implements fault-tolerant quantum gates. This direct control paves the way for such braiding schemes.

Looking ahead, the team aspires to scale their quantum dot arrays into more elaborate configurations, particularly a T-shaped structure comprising six quantum dots. Such geometries would permit not only movement but actual swapping—or braiding—of Majorana quasiparticles, a foundational step towards implementing topological qubits. Goswami cautions that the initial qubits built from these architectures may not yet rival other qubit platforms in performance, but they offer unmatched opportunities to investigate the fundamental quantum properties and interactions of Majorana modes, knowledge that is critical for future quantum technologies.

What excites Goswami most is not solely the eventual construction of a quantum computer but the fundamental exploration of Majorana physics itself. Unraveling the intricate ways these quasiparticles interact and couple could reveal novel phenomena and guide the engineering of more robust quantum devices. “We are peeling back the layers on how Majoranas behave, which could revolutionize quantum technology,” he reflects. This paradigm shift underscores the transition from hunting elusive signatures in messy materials toward constructing clean, tunable artificial lattices that faithfully realize topological models.

The implications of this work extend beyond quantum computing. The controlled realization of Kitaev-like chains with artificial atoms could serve as versatile quantum simulators for complex phenomena in condensed matter physics, enabling studies of topological phases and emergent excitations inaccessible in natural materials. Integration with advanced measurement protocols will further illuminate the dynamics of Majorana bound states, enriching our understanding of quantum matter. As the field advances, the interplay between theory and increasingly precise experiments will be crucial.

In sum, the Delft team’s achievement in creating a deterministic, tunable three-site Kitaev chain using quantum dots represents a transformative advance in topological quantum research. By blending sophisticated nanofabrication, cryogenics, and quantum control techniques, they have realized a pristine testbed to probe, manipulate, and ultimately harness Majorana bound states. This work bridges elegant theoretical constructs with tangible experimental implementation, bringing topological quantum computing one step closer to realization. The capacity to engineer such minimal yet complex systems is a harbinger of the next generation of quantum devices that capitalize on the strange and powerful properties of topological matter.

As quantum technologies race toward scalability and error correction, this landmark experiment shows that precision engineering of fundamental quantum models, rather than relying on imperfect natural materials, can yield reliable and tunable platforms for exotic quasiparticles. The continued development of these artificial Kitaev chains promises to unlock deeper insights into fault-tolerant quantum information processing and to inspire new architectures exploiting topological protection. The enduring quest to tame quantum error by mastering Majorana physics is clearly gaining ground, fueled by ingenious experimental creativity and relentless theoretical inquiry.

Subject of Research: Not applicable

Article Title: Observation of edge and bulk states in a three-site Kitaev chain

News Publication Date: 30-Apr-2025

Web References:
http://dx.doi.org/10.1038/s41586-025-08892-5

Image Credits: Picture by QuTech

Keywords: Qubits

The research was mainly carried out by Tuomas Ollikainen, a PhD student at Aalto university who split his time between carrying out experimental work in Amherst in Massachusetts, and analyzing the data and developing his theories at Aalto.

‘We hadn’t been able to study the dynamics of these sorts of three-dimensional structures experimentally before, so this is the first step to this direction.’ says Ollikainen. ‘The fact that the knot decays is surprising, since topological structures like quantum knots are typically exceptionally stable. It’s also exciting for the field because our observation that a three-dimensional quantum defect decays into a one-dimensional defect hasn’t been seen before in these quantum gas systems’

Controlling quantum gasses

The researchers hope their new study opens up new avenues in experimental research. One of the key breakthroughs in the study was being able to have better control over the state of the quantum gas, which allowed them to detect changes in its structure, like the decay of the knots and the formation of the vortex.

‘Of course one can simulate these things but actually making quantum knots is not that easy. By being able to control the environment better we can explore different effects and get to understand more about these exciting quantum systems.’ tells Ollikainen.

‘When we tied quantum knots in 2016, it was the first realization of three-dimensionally winding topological structures. That was like breathing air another planet for the first time. Amazing.’ says Prof. Mikko Möttönen, head of Quantum Computing and Devices group where Ollikainen works.

‘I know that many researchers have paid attention to our work and got inspiration to try this out in completely different type of systems. It would be great to see this technology being used some day in a practical application, which may well happen. Our latest results show that while quantum knots in atomic gases are exciting, you need to be quick to use them before they untie themselves. Thus the first applications are likely to be found in other systems.’ Möttönen continues.