A Dual-Purpose Quantum Protocol for Secure Communication and Heisenberg-Limited Remote Sensing Using Shared Entanglement

The application of quantum technologies is rapidly transitioning from theory to practice, with research groups worldwide working to bridge the gap between laboratory demonstrations and functional, deployable systems. Among the most active areas of development are quantum communication and quantum sensing—two domains that leverage the non-classical features of quantum mechanics, such as entanglement and superposition, to unlock capabilities that far exceed those of classical systems. Quantum communication offers robust, provable security rooted in the laws of physics, while quantum sensing enables extremely sensitive measurements of physical parameters, often with resolution down to the quantum limit. Despite these shared foundations, the two areas have traditionally been explored and implemented independently, each with its own set of protocols, hardware, and resource requirements. This separation not only leads to inefficiencies—especially in how quantum entanglement is consumed—but also creates unnecessary complexity when scaling these technologies toward larger, more integrated quantum networks. One of the key bottlenecks facing this next generation of quantum infrastructure is the inability to combine different quantum functionalities into a single streamlined protocol. Quantum sensing often demands highly entangled states that are difficult to produce and fragile under noise, while secure quantum communication requires elaborate error correction and eavesdropping detection methods. Merging these functions in a meaningful way is a nontrivial challenge. Questions about cross-interference between tasks, potential performance degradation, and the limitations of current quantum hardware—particularly in the NISQ (noisy intermediate-scale quantum) era—all contribute to the complexity of this problem.

In response to these challenges, a team of researchers led by Professor Gui-Lu Long and Dr. Dong Pan at Tsinghua University and Beijing Academy of Quantum Information Sciences (BAQIS), including PhD candidate Yu-Chen Liu and collaborators Yuan-Bin Cheng, Dr. Xing-Bo Pan, Ze-Zhou Sun, proposed a new approach to tackling this divide. In their recently published paper in Physical Review Applied, they introduced a unified framework that rethinks the relationship between quantum sensing and communication. Rather than treating them as separate functions, they developed a protocol that enables both tasks to be carried out simultaneously, using the same set of entangled photon pairs. Their goal was to not only conserve quantum resources, but to pave the way toward more scalable, multi-purpose quantum systems. To explore the feasibility of this idea, the team designed and simulated a complete protocol from start to finish. The system is built on entangled Bell pairs, which are distributed between two users—conventionally called Alice and Bob. What makes the protocol novel is that both quantum sensing and secure communication occur through manipulation of the same entangled states. The protocol includes all the essential elements: quantum state preparation, two layers of eavesdropping detection, encoding of classical information, phase sensing via controlled interactions with an unknown parameter, and subsequent quantum measurement.

The experimental design centered on a modified two-step quantum secure direct communication (QSDC) protocol. Here, Alice applies one of two unitary operations to encode a bit value, then subjects her qubits to a multi-pass phase estimation routine. These altered qubits are sent to Bob, who performs measurements using two different observables—σₓ ⊗ σₓ and σᵧ ⊗ σₓ—to extract both the encoded bit and the phase shift applied during sensing. To address the limited estimation range [0, 2π/N), the team employed a mixed-pass scheme, using both single and multiple photon interactions to recover the full range [0, 2π). The quantum Cramér-Rao bound theory revealed that this integrated scheme could approach Heisenberg-limited precision, meaning the variance of the phase estimation scales as 1/N² with , where N is the number of times the qubit interacts with the system containing the unknown phase—reaching the theoretical maximum allowed by quantum mechanics. This level of performance is usually only accessible via more complex entangled states, such as GHZ or NOON states, which are notoriously difficult to generate and maintain. By contrast, the QISAC protocol achieved this sensitivity using only bipartite entanglement, a much more experimentally accessible resource. Just as importantly, the protocol demonstrated resilience to attacks such as man-in-the-middle interceptions and double CNOT operations, keeping the quantum bit error rate (QBER) well within acceptable thresholds for secure communication.

The authors also studied how the system behaves under realistic constraints, such as a finite number of entangled pairs or limited measurement rounds. It shows that the estimation bias stays below the standard deviation for most θ values with limited entanglement resources. This confirms the QISAC protocol’s robustness, ensuring sensing accuracy alongside secure communication. This kind of flexibility suggests that the protocol can be fine-tuned for different priorities—whether that’s maximizing sensing resolution or enhancing message security. The team simulated the variation of θ with N for different numbers of EPR pairs, identifying an optimal N that demonstrates a quantum advantage in variance. The results confirm the estimation accuracy for this optimal N, showing improved bias reduction with more EPR pairs. Moreover, one of the most compelling aspects of the study was the trade-off analysis between these two goals. As more entangled pairs were devoted to sensing, the estimation precision improved, but the probability of detecting Eve successfully diminished. By mapping out this relationship, the researchers provide a practical framework that future users can adapt depending on their specific use case—be it secure data transmission, remote metrology, or hybrid applications.

What makes this study particularly meaningful is how it challenges the traditional boundaries in quantum system design. Rather than treating secure communication and high-precision sensing as two separate objectives, the authors propose a single protocol that handles both simultaneously. This approach addresses a longstanding inefficiency in quantum information science—the need to allocate separate entangled resources for distinct functions. Remarkably, the researchers achieve this integration without sacrificing performance. In fact, their protocol reaches the Heisenberg limit in sensing precision, all while maintaining a level of security compatible with established standards in quantum direct communication. It’s not often that such a dual-purpose solution emerges without major trade-offs, making this work a rare example of true optimization.

The impact of this work goes beyond theory. It offers a practical pathway toward building the next generation of quantum networks—networks where the same physical infrastructure can be used both for secure data transmission and for high-resolution environmental sensing. This dual capability has real potential in areas such as defense communications, satellite-based monitoring, and remote detection systems for phenomena like gravitational waves or seismic activity. In all these scenarios, accuracy and security are both essential, and this protocol manages to deliver on both fronts using a single entanglement resource. One of the more exciting aspects is the protocol’s efficiency. By allowing entangled pairs to serve double duty, the system reduces the size and cost of the required quantum hardware. In a field where scaling up remains a significant technical hurdle, this kind of resource-conscious design could help push quantum technologies closer to mainstream adoption. What’s more, the protocol is highly adaptable. It relies on bipartite entanglement—specifically Bell pairs—which are much easier to generate and maintain compared to more complex entangled states like GHZ or NOON states. That choice alone makes the approach far more accessible for near-term implementation using existing technologies. The simulations included in the study add even more weight to its practicality. They account for environmental noise, imperfect equipment, and realistic constraints on entanglement supply—all of which are unavoidable in real-world scenarios. Yet, even under those limitations, the protocol performs well, which suggests it isn’t just an idealized concept but something that could actually be deployed. What’s perhaps most compelling is the long-term vision this work supports. The QISAC framework could become a foundational element of the future quantum internet—an infrastructure where quantum nodes aren’t limited to one function but can communicate securely while simultaneously sensing their environment. Such a system would be faster, more secure, and arguably more intelligent. This research doesn’t just propose a protocol; it moves us meaningfully closer to that broader, integrated quantum future.