Glaucia Murta

Secure communication is a core task in cryptography which is focused on protecting the content of a message against eavesdroppers. In certain scenarios, however, the identities of the communicating parties are themselves sensitive and must remain concealed. This calls for anonymous communication protocols that ensure not only message secrecy but also the privacy of participants' identities. In this talk, I will present recent advances showing how quantum systems can offer advantages for anonymous communication in network settings. I will introduce a security framework that encompasses the secrecy of a shared key and the anonymity of the communicating users and show how the correlations of multipartite Greenberger-Horne-Zeilinger (GHZ) states enable protocols that outperform those based on bipartite entanglement. I will also highlight a recent experimental realization that demonstrates that these advantages are already accessible with current technology. Finally, I will outline open questions and possible directions for future research in this emerging area.

Quantum key distribution (QKD) enables the exchange of cryptographic keys with information-theoretic security. However, real-world implementations of QKD are often limited by noise, losses, and imperfect devices. High-dimensional quantum systems offer a promising route to overcome these limitations, enabling denser information encoding and enhanced resilience to noise and loss compared to traditional qubit-based protocols. On the other hand, device-independent (DI) protocols can address all adversarial cases due to device imperfections; however, existing security proofs have not shown any advantage associated with higher dimensions. In this work, we present a robust high-dimensional one-sided DI-QKD (1sDI-QKD) protocol whose security is certified through violations of quantum steering inequalities. By relaxing assumptions on one of the parties while still leveraging high dimensions, this approach improves the practicality of the protocols over fully device-independent QKD, offering promising experimental implications. We investigate 1sDI-QKD utilizing high-dimensional entanglement systems based on Srivastav et al. [PhysRevX.12.041023] framework and show that reverse reconciliation leverages the inherent asymmetry of the steering scenario, resulting in significantly higher key rates in the asymptotic regime as we increase the dimensions. Furthermore, we analyze the protocol's robustness to depolarizing noise and detection inefficiencies. Our results demonstrate the enhanced noise robustness and loss tolerance of high-dimensional 1sDI-QKD. The next step will be to experimentally validate these advantages, establishing high-dimensional 1sDI-QKD as a strong candidate for secure quantum communication under realistic conditions.

Here we report on the experimental results implementing robust anonymous quantum conference key agreement using GHZ states. Results confirm the advantage when allowing for the use of multipartite entanglement along with bipartite entanglement.

When the outcomes of a set of parties measuring their local quantum systems exhibit non-local correlations by violating a Bell inequality, one can infer that such outcomes are secret to some extent. This is at the core of the security of many device-independent (DI) protocols, such as DI randomness expansion and DI conference key agreement. We quantify the amount of secret randomness in the parties’ outcomes by analytically computing their conditional von Neumann entropies as a function of the Bell violation, for different Bell inequalities.

High-dimensional entanglement promises to increase the information capacity of photons and isnow routinely generated exploiting spatio-temporal degrees of freedom of single photons. A curiousfeature of these systems is the possibility to certify entanglement despite strong noise in the data.We show that it is also possible to exploit this noisy entanglement by introducing a protocol thatuses mutliple subspaces of the high-dimensional system simultaneously. Our protocol can be used toincrease key rates in realistic conditions. To that end, we conduct two simulations of our protocol fornoise models that apply to the two most commonly used sources of high-dimensional entanglement:time-bins and spatial modes.

We consider a device-independent scenario where N parties test the Mermin-Ardehali-Belinskii-Klyshko (MABK) inequality. By exploiting the inequality’s symmetries, we drastically simplify the general form of the quantum state that can be considered, without loss of generality. We then derive an upper bound on the maximal violation of the MABK inequality attained by an arbitrary N-qubit state, as a function of the state’s parameters. The two results enable us to derive analytical bounds on the von Neumann entropy of the parties’ outcomes, conditioned on the eavesdropper’s information. These quantities are crucial for the security of many cryptographic protocols and better bounds lead to more robust protocols. In particular, we bound the conditional entropy of a single party’s outcome and the joint conditional entropy of two parties’ outcomes, as a function of the MABK violation observed by three parties. We extend the former bound to N parties and prove its tightness, while we observe that the latter significantly improves previous results.