Four months ago, we were being persuaded that the Chinese have constructed a quantum radar, something that can inform you about an airplane without any actual reflection of any radiation from the airplane. This is obviously impossible by locality, whether you use any quantum subtleties or not.

— Lubos Motl

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You cannot know the moment of the collapse of the wavefunction by another observer, except when that observer tells you (by a classical channel).

— Me@2017-08-14 01:41:47 PM

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This point is irrelevant, since quantum radar is possible if there is somehow an interaction between the object and the entangled particle; destruction of the correlation informs the existence of a foreign object; although when exactly the correlation is destroyed, like when exactly the wavefunction collapses, is a physically meaningless question.

You cannot detect the change of correlation in real time. Instead, you need to get the information of the opposite side photons by a classical channel and then compare the photon pairs to get the correlation information.

— Me@2023-02-12 08:05:10 PM

— Me@2017-08-16 02:57:07 PM

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