Warning, speedsters: you can't fool quantum radar

Is it a bird, a plane or a speeding car? Police officers might one day be armed with speed guns that cannot be fooled by even the most determined racers, thanks to a new radar technique based on the laws of quantum mechanics. The ultra-secure system could also be used to counter futuristic military cloaking systems that might disguise a plane as a bird.

Radar and lidar systems bounce radio or light signals off an object and measure how long they take to return. That information can be used to determine the object's position and shape ? identifying it as a war plane, say ? or to calculate its speed. But both military and police systems can be fooled by devices that generate photons of the same frequency as in the outgoing beam. This is how the speed gun jammers installed in some cars work.

To reveal when returning photons have been faked, Mehul Malik and colleagues at the University of Rochester, New York, borrowed a trick from quantum cryptography, polarising each outgoing photon in one of two ways according to a sequence.

Wrong polarisation

Their radar system also measures the polarisations of the returning photons. That forces someone creating a false beam to polarise their photons too ? but they need to know the correct sequence. However, if they try to measure the photons arriving from the radar transmitter, quantum mechanics ensures that many of the true polarisations will get lost. So a false signal will always contain more photons of the wrong polarisations than the true beam.

In lab tests, photons reflected from a cut-out of a stealth bomber had an error rate of less than 1 per cent, while over half the photons created on the fly to mimic the shape of a bird had the wrong polarisation. In quantum cryptography, the same principle reveals if photons encoding a secret key have been intercepted.

"If it works in practice, it would be super-cool," says Vadim Makarov of the University of Waterloo, Canada. But he warns that reflected photons are more likely to change polarisation outside a laboratory setting.

Journal reference: Applied Physics Letters, doi.org/jz5

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