Their device, which can detect objects at a distance using only a few photons, raises the prospect of stealthy radar systems that emit little detectable electromagnetic radiation.

The researchers create pairs of entangled microwave photons using a superconducting device called a Josephson parametric converter. They beam the first photon, called the signal photon, toward the object of interest and listen for the reflection. In the meantime, they store the second photon, called the idler photon. When the reflection arrives, it interferes with this idler photon, creating a signature that reveals how far the signal photon has traveled.

We guess at that point they have to ask the cat what the entangled proton is doing.

Quantum radar significantly outperforms conventional radar at short distances. It also shows the potential application of quantum illumination more generally.

A big advantage is the low levels of electromagnetic radiation required. Then there is the obvious application as a stealthy radar that is difficult for adversaries to detect over background noise. The researchers say it could be useful for short-range low-power radar for security applications in closed and populated environments.

The paper can be found on arXiv.org.