We will discuss a tester formalism for the task of minimum-error channel discrimination that encompasses both standard parallel and sequential strategies and indefinite-causal-order strategies. We start by applying it to general channels, and discover an instance of a discrimination task of two qubit-qubit channels, an amplitude-damping and a bit-flip channel, for which a sequential strategy outperforms any parallel strategy.
In this talk I will discuss several protocols designed for the verification and the characterization of quantum devices. In the first part of the talk I will focus on the purely classical verification of the output of a quantum computer and will present a minimal example for realizing such a verification protocol with current technology. In the second part I present experimentally accessible conditions for detecting entanglement
The quantum marginal problem
Clarifying the relation between the whole and its parts
is crucial for many problems in science. In quantum
mechanics, this question manifests itself in the quantum
marginal problem, which asks whether there is a global pure
quantum state for some given marginals. This problem arises
in many contexts, ranging from quantum chemistry to entanglement
theory and quantum error correcting codes.
An interesting class of open quantum systems is defined by the so-called repeated interaction scheme, where the system interacts sequentially with small and fresh subsystems or units coming from the reservoir. The interaction is unitary and relatively easy to analyze. However, it must be switched on and off, and this action introduces or extracts energy in many cases of interest, performing work and preventing thermalization. As a consequence, the repeated interaction scheme cannot be used to model thermostats in quantum thermodynamics.
We show that the positivity of the Wigner function of Gaussian states and measurements provides an elegant way to bound the discriminating power of "linear optics", which we formalise as Gaussian measurement operations augmented by classical (feed-forward) communication (GOCC). This allows us to reproduce and generalise the result of Takeoka and Sasaki [PRA 78:022320, 2008], which tightly characterises the GOCC norm distance of coherent states, separating it from the optimal distinguishability according to Helstrom's theorem.