Antonio Acin

Device-independent self-testing refers to the certification of quantum states based entirely on the correlations exhibited by measurements on separate subsystems. The fact that such a certification is possible at all is remarkable in its own right, and is intimately connected to the violation Bell’s inequalities by entangled quantum systems. In the bipartite case, self-testing of states has been completely characterized, up to local isometries, as there exist protocols that self-test arbitrary pure states of any local dimension. Despite the growing interest in device-independent certification protocols, an analogous result in the general multipartite case has remained elusive. In this work, we give a complete characterization of the qubit case, showing that any multipartite entangled state of qubits can be self-tested.

Device-independent quantum key distribution (DIQKD) provides the strongest form of quantum security, as it allows two honest users to establish secure communication channels even when using fully uncharacterized quantum devices. The security proof of DIQKD is derived from the violation of a Bell inequality, mitigating side-channel attacks by asserting the presence of nonlocality. This enhanced security comes at the cost of a challenging implementation, especially over long distances, as losses make Bell tests difficult to conduct successfully. Here, we propose a photonic realization of DIQKD, utilizing a heralded preparation of a single-photon path entangled state between the honest users. Being based on single-photon interference effects, the obtained secret key rate scales with the square root of the quantum channel transmittance. This leads to positive key rates over distances of up to hundreds of kilometers, making the proposed setup a promising candidate for securing long-distance communication in quantum networks.

In this work, we study the consequences of entanglement-assistance (EA) for randomness generation in the semi-DI framework based on energy constraints introduced in (van Himbeeck et al., Quantum 1, 33 (2017)). We show that, given an energy bound $\omega$, the minimum min-entropy that non-EA honest devices can certify decreases when entanglement between the prepare and measurement boxes (of the devices prepared by Eve) is allowed.

Continuous variable quantum key distribution (CVQKD) with discrete modulation combines advantages of CVQKD, such as the implementability using readily available technologies, with advantages of discrete variable quantum key distribution, such as easier error correction procedures. In this work we consider a phase-shift keying protocol using four coherent states (4-PSK protocol) and coarse-grained heterodyne measurements. We provide a security proof against coherent attacks and compute the achievable key rate in a finite size setting, i.e. with a finite number of rounds. To this end, we employ the generalized entropy accumulation theorem, as well conic optimisation, providing us with improved key rates compared to previous works. At metropolitan distances our method can provide positive key rates for the order of $10^9$ rounds. We also provide a theoretical method to overcome the assumption of a finite photon number cutoff made in previous works.