  Nonreciprocal devices are a key element for signal routing and noise
isolation. Rapid development of quantum technologies has boosted the demand for
a new generation of miniaturized and low-loss nonreciprocal components. Here we
use a pair of tunable superconducting artificial atoms in a 1D waveguide to
experimentally realize a minimal passive nonreciprocal device. Taking advantage
of the quantum nonlinear behavior of artificial atoms, 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. Our
experiment highlights the role of quantum correlations in enabling
nonreciprocal behavior and opens a path to building passive quantum
nonreciprocal devices without magnetic fields.
