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From https://www.digitaltrends.com/cool-tech/quantum-radar-stealth-aircraft/

In principle, a quantum radar functions like a regular radar — only that instead of sending out a single beam of electromagnetic energy, it uses two split streams of entangled photons. Only one of these beams is sent out, but due to a quirk of quantum physics both streams will display the same changes, despite being potentially miles apart. As a result, by looking at the stream which remains back home it’s possible to work out what has happened to the other beam.

Wouldn't this imply faster-than-light information transfer? Is there any way that the interactions of a remote particle can influence a local entangled particle?


marked as duplicate by AccidentalFourierTransform, Kyle Kanos, Community Nov 16 '18 at 23:12

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Quantum radar is a real thing. The description is somewhat wrong.

In radar, you send out a beam of radio waves which get reflected, and you measure them when you come back to look at the objects they were reflected off of.

In quantum radar you send out a beam of radio waves, which are entangled with another beam. This first beam gets reflected. And when it comes back, you interfere them with the other beam to learn about the objects that reflected them.

So there's no faster-than-light transfer, because the beam comes back before you extract the information (just like conventional radar).

There are real theoretical advantages to using quantum radar, but it is also quite a bit more complicated to make it work. The article says that it's not clear whether this is a real device yet or whether it's still vaporware.

  • $\begingroup$ Where do they keep the 2nd beam while the 1st is our exploring the sky? Does it just keep bouncing around in a bottle somewhere? If the wavelength is kilometers long, wont it dissipate before the 1st gets back? $\endgroup$ – Chloe Nov 18 '18 at 19:16
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    $\begingroup$ @Chloe: this is presumably one of the things that makes building these things difficult. For a demonstration system, you could just bounce it off something you made specially for this purpose. For a real, portable, system maybe you could store it in optical fibers. (From wikipedia, it looks like you need to start with visible or near-infrared frequencies to produce the entanglement, after which you convert to microwave frequencies.) $\endgroup$ – Peter Shor Nov 18 '18 at 20:45

Yes. See https://en.wikipedia.org/wiki/Quantum_radar.

The particles retain their entanglement at large separations.

This question discusses the issue of whether this implies faster-than-light information transfer:

Quantum entanglement faster than speed of light?

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    $\begingroup$ Peter Shor’s answer is much better than mine, and I have voted for his. $\endgroup$ – G. Smith Nov 16 '18 at 19:34

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