[Cryptography] Spooky quantum radar at a distance

[Natanael]

Den 23 sep. 2016 8:12 em skrev "Jerry Leichter" <leichter at lrw.com>:
>
> >
http://www.scmp.com/news/china/article/2021235/end-stealth-new-chinese-radar-capable-detecting-invisible-targets-100km
> There is an important concept in quantum mechanics called "weak
measurements" https://en.wikipedia.org/wiki/Weak_measurement.  The original
presentation of these was the following problem:  Inside a box there may -
or may not - be a nuclear bomb.  The bomb is attached to a trigger that
will go off if the bomb absorbs even a single photon.  You want to
determine if there's a bomb in the box, but obviously you would rather not
set it off if it's there.  Can you do so?
>
> Bizarrely enough, the answer is yes - mainly.  You can trade off your
chance of accidentally triggering the bomb against the chance that the
answer you get to your question is wrong (beyond the classical extremes
of:  Don't test, never cause an explosion, give an arbitrary answer and be
right half the time; and always test, give an always-correct answer the
half of the times the bomb isn't there, blow up the other half.)

As you see in my other reply, I've mentioned this possibility. Also, this
can be improved upon to become effectively interactionless on average,
using the zeno effect as I also mentioned.

> Stealth aircraft have extremely small radar scattering cross-section:  If
you aim a radar at them, almost none of the energy gets reflect back to the
sender, so it can't "see" the plane.  There's some odd language in there
that kind of hints that you create pairs of correlated particles, send one
toward the plane, and then learn about what happens to it based on what
happens to its stay-at-home pair.

I read it differently. Instead they use the anti-correlation between the
photons in the pair to be able to identify the returning reflected photon
out of all the noise (since the rest of the detected photons will be
effectively randomized). At least that seems to be what they're implying.

You can read the local photon from the pair to see what its properties are,
and then configure your sensor to only look for photons with the exact
opposite properties. Then you look at the timing of the detection.

I'm not entirely sure though how the reflection against the target will
affect the properties of the photon, or how it affects the entanglement. An
actual physicist has to answer that.
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