Radar was a great invention that made air travel much safer and weather prediction more accurate, indeed it is even credited with winning the Battle of Britain. However, it carries a little problem with it during times of war. Painting a target with radar (or even sonar) is equivalent to standing up and wildly waving a red flag in front of your enemy, which is why for example submarines often run silent and only listen, or why fighter aircraft often rely on guidance from another aircraft. However, researchers in Italy, the UK, the US, and Austria have built a proof-of-concept radar that is very difficult to detect which relies upon quantum entanglement.
Despite quantum physics being hard to follow, the concept for the radar is pretty easy to understand. First, they generate an entangled pair of microwave photons, a task they perform with a Josephson phase converter. Then they store an “idle” photon while sending the “signal” photon out into the world. Detecting a single photon coming back is prone to noise, but in this case detecting the signal photon disturbs the idle photon and is reasonably easy to detect. It is likely that the entanglement will no longer be intact by the time of the return, but the correlation between the two photons remains detectable.
Of course, in reality they don’t use just a single photon. The process — known as quantum illumination — uses the weakened correlation and sampling to beat the performance of other techniques using the same number of photons and bandwidth.
Other than the quantum part, the rest of the setup is pretty simple to understand. The signal frequency is a little more than 10 GHz, while the idler is at almost 7 GHz. The receiver down converts to an intermediate frequency of 20 MHz and then uses a 100 MHz 8-bit A/D to grab the signal and do a Fourier transform.
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