[Cryptography] Spooky quantum radar at a distance

dj at deadhat.com dj at deadhat.com
Fri Sep 23 18:14:04 EDT 2016

> Den 22 sep. 2016 23:07 skrev "Henry Baker" <hbaker1 at pipeline.com>:
>> 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.

Not a change, there's no transmission at that point. The measurement of
one particle will be correlated with the measurement of the other
particle. This is the basis of QKD - you can yield the same random value
at both ends. It's not FTL because you had to send the particle on it's
way. It's not transmission of information yielded from the measurement
because the information didn't exist at either end until measured.

>> 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's exactly how normal radar works. Measurement and entanglement are
the same thing. It's not that effective though.

>> 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.

I don't see how that could work. There's a rather limited amount of
information and the entanglement of the remote particles can't be used to
transmit any information. You would need to send lots of photons and keep
track of them all in order to get one back. Keeping track of one photon
right now required special handling.

> 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

What's to prevent the bomb including a device that implements a hermitian
matrix that's the inverse of the hermitian matrix that represents the path
of the photon to the bomb so making the photon sharp (in quantum
terminology) at the bomb?

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