📝 SummarXiv

Abstract

Device-independent quantum key distribution (DI-QKD) leverages nonlocal correlations to establish cryptographic keys between two honest parties while making minimal assumptions about the underlying systems. The security of DI-QKD is grounded in the validity of quantum theory, with Bell violations ensuring the intrinsic unpredictability of observed statistics, independent of the trustworthiness of the devices. While traditional collective QKD attacks involve an adversary preparing the shared system for the users, we analyse a scenario in which the adversary does not control the preparation devices yet can still extract information about the key using a strategy known as the sequential attack. In this approach, an eavesdropper (Eve) intercepts the transmitted quantum particle, performs an unsharp measurement to maintain the Bell violation between the honest parties, and disguises her interference as noise. This method enables Eve to acquire relevant information about the key without resorting to collective measurements. We show that by using sequential measurements with an ancilla stored in quantum memory, Eve can mimic a collective attack within certain parameter ranges, even without initial control of the source.