  Quantum illumination is a powerful sensing technique that employs entangled
signal-idler photon pairs to boost the detection efficiency of low-reflectivity
objects in environments with bright thermal noise. The promised advantage over
classical strategies is particularly evident at low signal powers, a feature
which could make the protocol an ideal prototype for non-invasive biomedical
scanning or low-power short-range radar. In this work we experimentally
investigate the concept of quantum illumination at microwave frequencies. We
generate entangled fields using a Josephson parametric converter to illuminate
a room-temperature object at a distance of 1 meter in a free-space detection
setup. We implement a digital phase conjugate receiver based on linear
quadrature measurements that outperforms a symmetric classical noise radar in
the same conditions despite the entanglement-breaking signal path. Starting
from experimental data, we also simulate the case of perfect idler photon
number detection, which results in a quantum advantage compared to the relative
classical benchmark. Our results highlight the opportunities and challenges on
the way towards a first room-temperature application of microwave quantum
circuits.
