[ExI] Quantum Radar

[Stuart LaForge]

Cool new tech: Quantum Radar. A team of scientists at the Institute of  
Science and Technology in Austria just invented a cool new kind of  
radar. It uses quantum entangled pairs of microwave photons to  
drastically reduce the power consumption of radar and also makes radar  
surveillance difficult to detect. This is one of the coolest practical  
applications quantum entanglement that has been rendered into practice.

https://www.technologyreview.com/s/614160/quantum-radar-has-been-demonstrated-for-the-first-time/
https://arxiv.org/abs/1908.03058

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The device is simple in essence. 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. Voila—quantum radar!

This technique has some important advantages over conventional radar.  
Ordinary radar works in a similar way but fails at low power levels  
that involve small numbers of microwave photons. That’s because hot  
objects in the environment emit microwaves of their own.

In a room temperature environment, this amounts to a background of  
around 1,000 microwave photons at any instant, and these overwhelm the  
returning echo. This is why radar systems use powerful transmitters.

Entangled photons overcome this problem. The signal and idler photons  
are so similar that it is easy to filter out the effects of other  
photons. So it becomes straightforward to detect the signal photon  
when it returns.

Of course, entanglement is a fragile property of the quantum world,  
and the process of reflection destroys it.  Nevertheless, the  
correlation between the signal and idler photons is still strong  
enough to distinguish them from background noise.

This allows Barzanjeh and co to detect a room temperature object in a  
room temperature environment with just a handful of photons, in a way  
that is impossible to do with ordinary photons. “We generate entangled  
fields using a Josephson parametric converter at millikelvin  
temperatures to illuminate a room-temperature object at a distance of  
1 meter in a proof of principle radar setup,” they say.

The researchers go on to compare their quantum radar with conventional  
systems operating with similarly low numbers of photons and say it  
significantly outperforms them, albeit only over relatively short  
distances.

That’s interesting work revealing the significant potential of quantum  
radar and a first application of microwave-based entanglement. But it  
also shows the potential application of quantum illumination more  
generally.

A big advantage is the low levels of electromagnetic radiation  
required. “Our experiment shows the potential as a non-invasive  
scanning method for biomedical applications, e.g., for imaging of  
human tissues or non-destructive rotational spectroscopy of proteins,”  
say Barzanjeh and co.

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.
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Other applications come to mind such as detecting concealed weapons in  
crowded environments without having to funnel people through a scanner.

Stuart LaForge