A quantum radar using entangled microwave photons has been created at the Institute of Science and Technology Austria.

Also known as ‘microwave quantum illumination’, the demonstration detected objects in a noisy thermal environment – and there are potential applications for it in low-power biomedical imaging and security scanners, according to the Institute.

“What we have demonstrated is a proof of concept for microwave quantum radar,” said researcher Shabir Barzanjeh. “Using entanglement generated at a few thousandths of a degree above absolute zero, we have been able to detect low-reflectivity objects at room-temperature.”

Instead of using conventional microwaves, the researchers entangle two groups of photons – ‘signal’ and ‘idler’ photons.

Signal photons are sent out towards the object of interest, whilst the idler photons are measured locally.

By the time the signal photons are reflected back, they have lost true entanglement with the idler photons, but some correlation survives, according to the Institute, creating a signature that describes the existence or the absence of the target object, irrespective of environmental noise.

A cryogenic Josephson parametric converter (JPC) is used to create entanglement in the set-up, and the experiment relies on linear quadrature measurements and suitable post-processing to compute all covariance matrix elements from the full measurement record, according to the paper ‘Microwave quantum illumination using a digital receiver’, published in Science Advances. This enables an implementation of the phase-conjugate receiver that fully exploits the correlations of the JPC output fields without analogue photo-detection.

“At low power levels, conventional radar systems typically suffer from poor sensitivity as they have trouble distinguishing the radiation reflected by the object from naturally occurring background radiation noise,” according to IST Austria. “Quantum illumination offers a solution to this problem as the similarities between the signal and idler photons makes it more effective to distinguish the signal photons from the noise generated within the environment.”

“The main message behind our research is that quantum radar is not only possible in theory but also in practice,” said Barzanjeh, now at the University of Calgary. “When bench-marked against classical low-power detectors in the same conditions we already see, at very low-signal photon numbers, that quantum-enhanced detection can be superior.”

According to fellow researcher Professor Johannes Fink: “This result was only possible by bringing together theoretical and experimental physicists that are driven by the curiosity of how quantum mechanics can help to push the fundamental limits of sensing. But to show an advantage in practical situations we will also need the help of experienced electrical engineers and there still remains a lot of work to be done in order to make our result applicable to real-world detection tasks.”

IST Austria worked with the Massachusetts Institute of Technology, the UK University of York and the University of Camerino, Italy.

The Science Advancespaper is here‘, and available full free of charge.