Xavier Valcarce

The security of finite-length keys is essential for the implementation of device-independent quantum key distribution (DIQKD). Presently, there are several finite-size DIQKD security proofs, but they are mostly focused on standard DIQKD protocols and do not directly apply to the recent improved DIQKD protocols based on techniques such as noisy preprocessing and random key measurements. Here, we provide a general finite-size security proof that can simultaneously encompass these approaches, using tighter finite-size bounds than previous analyses. In doing so, we develop a method to compute tight lower bounds on the asymptotic keyrate for any such DIQKD protocol with binary inputs and outputs. With this, we show that positive asymptotic keyrates are achievable up to depolarizing noise values of 9.26%, exceeding all previously known noise thresholds. Furthermore, we also consider in greater detail a particular form of generalized CHSH inequality, and derive partial closed-form results for such cases. We discuss the potential advantage of this approach for realistic photonic implementations of DIQKD.

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.

Quantum Key Distribution (QKD) enables the expansion of cryptographic keys between two parties, allowing for proven secure communication. The main downside of QKD protocols is their vulnerability to attacks that target the physical implementation. Device Independent Quantum Key Distribution (DIQKD) is a new paradigm addressing this issue by relaxing assumptions on the physical implementation. First DIQKD experiments were reported in 2022, proving the feasibility of DIQKD. However, these experiences required highly sophisticated setups. Here, we analyse the suitability of a novel optical implementation for DIQKD. Our results show that DIQKD could be realized with a simple setup using only commercially available hardware.