Classical-Quantum Combs, their Min-Entropy and their Measurement-Based Applications

Learning a hidden property of a quantum system typically requires a series of interactions. In this work, we consider a formalisation of such multi-round learning processes that uses a generalisation of classical-quantum states called classical-quantum combs. Here, "classical" refers to a random variable encoding the hidden property to be learnt, and "quantum" refers to the quantum comb describing the behaviour of the system. By using the quantum combs formalism, the optimal strategy for learning the hidden property can be quantified via the comb min-entropy (Chiribella and Ebler, NJP, 2016). With such a tool on hand, we focus attention on an array of combs derived from measurement-based quantum computation (MBQC) and related applications. Specifically, we describe a known blind quantum computation (BQC) protocol using the combs formalism and thereby leverage the min-entropy to provide a proof of single-shot security for multiple rounds of the protocol, extending the existing result in the literature. Furthermore, we introduce novel connections between MBQC and quantum causal models and quantum causal inference, which allows for the use of the min-entropy to quantify the optimal strategy for causal discovery. We consider further operationally motivated examples, including one associated to learning a quantum reference frame.