Nonreciprocal devices are a key element for signal routing and noise isolation. The latest developments in the field of quantum technologies have boosted the demand for a new generation of miniaturized and low-loss nonreciprocal components. In this talk, I will present our results on how to use a pair of tunable superconducting artificial atoms embedded in a rectangular waveguide to experimentally realize a minimal, passive, nonreciprocal device. Taking advantage of the quantum nonlinear behavior of superconducting qubits, we achieve nonreciprocal transmission through the waveguide in a wide range of powers. Our results are consistent with theoretical modeling showing that nonreciprocity is associated with the population of the two-qubit nonlocal entangled quasi-dark state, which responds asymmetrically to incident fields from opposing directions. Additionally, our waveguide QED implementation allows us to explore other interesting phenomena, such as atom-photon bound states and entanglement stabilization.