Laura dos Santos Martins

Quantum sensors are powerful tools for measuring physical quantities with high sensitivity, enabling, for instance, the mapping of Earth’s gravitational field , detecting very small changes of magnetic fields, or the passage of time. The underlying principle is to use a quantum state as a probe that interacts with the physical quantity of interest, thereby encoding relevant information into the state. Although individual quantum sensors may exhibit remarkable sensitivity, the precision of a certain measurement can be significantly enhanced when multiple probes are entangled. Distributed quantum sensing extends this further and leverages entanglement among spatially separated sensors, allowing them to function as a single, coherent system. This approach enables measurements across extended spatial regions, while surpassing the precision achievable by independent sensors. However, a significant challenge in a network setting is ensuring that sensors deployed across different parties serve as the necessary resources for the correct functioning of the target sensing task. This challenge has motivated the combination of quantum cryptography with quantum sensing. In this context, Shettell et al. introduced the notion of privacy for sensor networks, ensuring that, beyond the metrological advantage of cooperative estimation of a global function, parties can also maintain the privacy of their local information and control what data is accessible to others. In this work, we adopt this protocol and focus on a multi-user quantum sensor network framework to analyze the privacy aspects of this parameter estimation task, leveraging a high-quality four-party GHZ state source.

Authentication of quantum resources is a critical tool in the development of quantum information processing protocols. In particular, the verification of quantum states is often used as a building block for communication tasks, determining whether the communicating parties can trust the resources at hand to exchange information or whether the protocol should be aborted. Self-testing methods have been used to tackle such verification tasks in a device-independent (DI) scenario. However, these approaches commonly consider the limit of large, identically and independently distributed (IID) samples, which weakens the DI claim and poses serious challenges to their experimental implementation. To address these issues, Gocanin et al. [1] developed a protocol to certify quantum states in the few-copies and non-IID regime. In this work, we adopt their protocol to experimentally demonstrate the device-independent verification of a four-photon GHZ state, produced with our compact and high-fidelity multipartite entangled photon source.

Quantum transmission links are central elements in essentially all implementations of quantum information protocols. Emerging progress in quantum technologies involving such links needs to be accompanied by appropriate certification tools. In adversarial scenarios, a certification method can be vulnerable to attacks if too much trust is placed on the underlying system. Here, we propose a protocol in a device independent framework, which allows for the certification of practical quantum transmission links in scenarios where minimal assumptions are made about the functioning of the certification setup. We take in particular unavoidable transmission losses into account by modeling the link as a completely-positive trace-decreasing map. We also crucially remove the assumption of independent and identically distributed samples, which is known to be incompatible with adversarial settings. Finally, in view of the use of the certified transmitted states for follow-up applications, our protocol allows to estimate the quality of the state and does not certify the channel only. To illustrate the practical relevance and the feasibility of our protocol with currently available technology we provide an experimental implementation based on a state-of-the-art polarization entangled photon pair source in a Sagnac configuration and analyse its robustness for realistic losses and errors.