In quantum entanglement, two or more particles are seemingly linked in a state of mutual instantaneous influence, no matter how far apart they are. And its not just quantum computers that make use of entangled particles these days. Everything from atomic clocks and unbreakable codes to the magnetic navigation abilities of birds can all exploit these effects. The latest application uses correlations between entangled microwave and optical beams to detect objects of low reflectivity — creating in effect, quantum radar.

The entangled particles in a quantum computer are typically ionized atoms that act as qubits, while for the avian retina the particles are electrons that form a so-called radical pair. To make a quantum radar, the particles needed are entangled photons in the microwave band. Researchers led by Stefano Pirandola have proposed that this could be achieved by interconverting microwave and optical signals using a device called an electro-optomechanical converter.

Such a device would accommodate each kind of radiation in cavities, separated by a vibrating membrane that entangles the optic and microwave fields. For any given transmission energy, this kind of quantum illumination setup would be more efficient than a classical microwave radar. The authors claim their system would be ideal for low-reflectivity objects embedded in a bright thermal background. A radar like this would come in handy for detecting stealthy targets that barely stand out against the background of the sky.

From their description the design sounds reminiscent of a basic interferometer system that uses a probe beam and a reference beam. The converter creates the microwave probe beam that would illuminate the object to be detected, while the visible arm acts as the reference beam — or ‘idler beam’ as they call it. The two beams then interfere with each other in the detector.

The idea of quantum illumination was first conceptualized back in 2008 by Seth Lloyd (of quantum computer fame). In 2013 Pirandola and his colleagues first demonstrated that it works for visible light. Getting the technique to work in at the longer microwave regime, however, still has many challenges. For the radio range, even longer still, there are plenty of other interesting applications. While we generally don’t associate an MRI with the kinds of ionizing radiation we expect in something like a CAT scan, for imaging delicate protein or nucleic acid samples, low power NMR would be very useful.

Similarly, the advantages of illuminating your target with the absolute minimum amount of energy required to identify it increases the chance that you yourself will not in turn be given away. Although we might say that quantum radar is still in Kickstarter stage, capabilities like that may soon draw some high-profile backers.