Jean-Daniel Bancal

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.

Sources producing high amounts of entanglement are essential for quantum cryptography. Given an uncharacterized source, manufactured by a possibly untrusted entity, how can we certify that it produces a lot of entanglement? We initiate the study of operational device-independent entanglement certification by presenting a device-independent protocol that lower-bounds the one-shot distillable entanglement of the remaining quantum state after the execution of the protocol. By this, the protocol certifies the amount of “useful entanglement” available for proceeding applications. Importantly, our protocol does not abort, with high probability, when testing realistically noisy sources.

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.

We investigate a class of partially device-independent quantum key distribution protocols based on a prepare-and-measure setup which simplifies their implementation. The security of the protocols is based on the assumption that Alice’s prepared states have limited overlaps, but no explicit bound on the Hilbert space dimension is required. The protocols are therefore immune to attacks on Bob’s device, such as blinding attacks. The users can establish a secret key while continuously monitoring the correct functioning of their devices through observed statistics. We report a proof- of-principle demonstration, involving mostly off-the-shelf equipment, as well as a high-efficiency superconducting nanowire detector. A positive key rate is demonstrated over a 4.8km low-loss optical fiber with finite-key analysis. The prospects of implementing these protocols over longer distances is discussed.

Device-independent quantum key distribution provides security even when the equipment used to communicate over the quantum channel is largely uncharacterized. An experimental demonstration of device-independent quantum key distribution is however challenging. A central obstacle in photonic implementations is that the global detection efficiency, i.e., the probability that the signals sent over the quantum channel are successfully received, must be above a certain threshold. We here propose a method to significantly relax this threshold, while maintaining provable device-independent security. This is achieved with a protocol that adds artificial noise, which cannot be known or controlled by an adversary, to the initial measurement data (the raw key). Focusing on a realistic photonic setup using a source based on spontaneous parametric down conversion, we give explicit bounds on the minimal required global detection efficiency.

Violation of a Bell inequality can be used to generate certified random numbers. Given the high rate at which pairs of entangled photons can be produced, they constitute promising candidates for high rate randomness generation. However, this requires closing the detection loophole. Here we present our progresses toward an experimental demonstration of randomness generation with photons, certified by the violation of a Bell inequality with a closed detection loophole.