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The Department of National Defence is investing into a technology they are coining quantum radar. The premise is that they would create an entangled pair of photons, and shoot one out into the air while monitoring the other one.

Before asking a question here a few years ago about what/how quantum entanglement is/works, my hope was that entangled particles could work this way, because if this is how entanglement worked, one could achieve faster than light communication.

But my understanding of entanglement now has been clarified to be more of a shared secret, that it may have uses in assuring authenticity, or detection of eavesdropping, specifically because the entangled particles share a secret and are not locked together. That is, changing the state of an entangled particle does not change the state of it's partner, that it simply decouples them to no longer be entangled.

However the descriptions of these proposed 'quantum radar' now makes me question yet again if I have an accurate understanding of quantum entanglement.

So I guess my question is, yet again, does changing the state/information of an entangled particle have any effect at all on it's partner(s)?

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To answer your final question, no. There is no measurement you can make on your local photon that will tell you anything about what has happened to the remote photon.

The idea of the project you've mentioned is not to shoot one into the air while monitoring just the other one. Big picture, they want to shoot one into the air, and then, if it reflects off of something (like a stealth plane), detect it and measure both photons.

By looking at correlations between the incoming photons and the photons that remained in the lab, they can more easily separate background noise (which was never entangled with your photons, and so should be uncorrelated) and signal (which still has some correlations with your photons).

Relevant paper on this topic: Quantum Illumination.

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  • $\begingroup$ Yes, an elastic scatter would keep the phases. $\endgroup$ – anna v May 18 '18 at 8:11

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