[Cryptography] Spooky quantum radar at a distance

[Natanael]

Den 22 sep. 2016 23:07 skrev "Henry Baker" <hbaker1 at pipeline.com>:
>
> FYI --
>
>
http://www.scmp.com/news/china/article/2021235/end-stealth-new-chinese-radar-capable-detecting-invisible-targets-100km
>
> The end of stealth?  New Chinese radar capable of detecting 'invisible'
targets 100km away

[...]

> Quantum physics says that if you create a pair of entangled photons by
splitting the original photon with a crystal, a change to one entangled
photon will immediately affect its twin, regardless of the distance between
them.
>
> A quantum radar, generating a large number of entangled photon pairs and
shooting one twin into the air, would be capable of receiving critical
information about a target, including its shape, location, speed,
temperature and even the chemical composition of its paint, from returning
photons.
>
> That sounds similar to a normal radar, which uses radio waves, but
quantum radar would be much better at detecting stealth planes, which use
special coating materials and body designs to reduce the radio waves they
deflect, making them indistinguishable from the background environment.
>
> In theory, a quantum radar could detect a target's composition, heading
and speed even if managed to retrieve just one returning photon.  It would
be able to fish out the returning photon from the background noise because
the link the photon shared with its twin would facilitate identification.

If you think that's mind-bending, you should take a look at another one of
the even crazier examples of quantum physics in action - you can combine
the two principles of quantum counterfactual definiteness and the quantum
zeno effect to do the following;

https://web.archive.org/web/20160304114610/http://physics.illinois.edu/people/kwiat/interaction-free-measurements.asp

Detecting things with *one* photon isn't quite the same as detecting them
with *none*, interaction-free detection!

Although the big practical difference here is that this Chinese tech works
in free air since it relies on a normal 180° reflection off an object back
to your sensors (it behaves like you'd expect a normal radar to behave),
while this other quantum system relies on a fully controlled path (with
mirrors and optics all the way) where you just can detect if your measured
path is being obscured or not. The TL;DR: It relies on photon
self-interference being broken in a detectable manner. To paraphrase the
thread title, "spookier quantum detector in proximity".

The latter detector system system could be useful in many controlled
environments where you're able to provide a controlled path for it to
detect interruptions in. It would work anywhere you would normally use
something like laser detection but want to get rid of the light (and can
assure a single photon wouldn't be lost!).

Alarm systems and tamper detection are two obvious examples, and in the
latter case you could for example use a fiber optic cable to provide the
"dark" measured path which can't be broken undetected. Counting light
sensitive objects is another possibility, such as in a manufacturing line
in a dark room (items being counted as they pass by the pair of mirrors).

More: https://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb_tester
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.metzdowd.com/pipermail/cryptography/attachments/20160923/3c8b034b/attachment.html>