[Cryptography] Spooky quantum radar at a distance

[Watson Ladd]

On Sat, Sep 24, 2016 at 12:41 AM, John Denker <jsd at av8n.com> wrote:
> On 09/22/2016 09:31 AM, Henry Baker wrote:
>
>> http://www.scmp.com/news/china/article/2021235/end-stealth-new-chinese-radar-capable-detecting-invisible-targets-100km
>
> That article seems to be largely based on the following press release:
>   http://www.cetc.com.cn/zgdzkj/_300931/_300939/445284/index.html
>
> Note that the 100 km detection claim appears in the CETC press release,
> while the flamboyant references to Einstein and to stealth do not.
> Perhaps they were added by the SCMP journalist.
>
>>  The end of stealth?  New Chinese radar capable of detecting
>> 'invisible' targets 100km away
> ...
>> The breakthrough relies on a ghostly phenomenon known as quantum
>> entanglement, which Albert Einstein dubbed "spooky action at a
>> distance".
>
> The article is so deep in hogwash that one hardly knows where to begin.
>
>  a) quantum mechanics does exhibit entanglement, and
>  b) even though Einstein ridiculed the idea of action at a distance,
>   quantum mechanics actually does exhibit a certain type of nonlocality.
>
> However, those are not the same thing.  Really not.  Also, the nonlocality
> cannot be used to transmit information.
>
> Meanwhile:
>  c) There is such a thing as "quantum illumination".  It makes use
>   of entanglement.  It does not even attempt to exploit nonlocality,
>   and any such attempt would fail anyway, because EPR-type nonlocality
>   does not transmit information.  Instead, quantum illumination relies
>   on a beam that goes out and comes back, whereupon it is compared
>   against a local reference (the "ancilla") that has been retained,
>   locally.
>
> Meanwhile:
>>  The end of stealth?
>
> We can invoke Betteridge's law of headlines:  Any headline that ends
> in a question mark can be answered "no".
>
> The stealth idea has been circling the drain for quite a while now, for
> reasons having nothing to do with quantum illumination.
>
>   d) As Peter G. rightly pointed out, all existing stealth aircraft show
>    up on S-band and L-band radar, and on any longer wavelength.
>
>   e) To which one might add:  Any stealth aircraft will show up on
>    bistatic and multistatic radar.  The aircraft scattering function
>    has a null in the direct backscatter direction, directly back toward
>    the transmitter.  However, physics does not permit it to be null in
>    every direction.  There *will* be a pattern of nodes and antinodes.
>
>    If only they had an outpost in the South China Sea, it would make
>    a great site for a multistatic radar installation.  Oh, wait, maybe
>    they already thought of that.......
>
>   f) Stealth aircraft also show up in the infrared.
>
> This bag has been catless for many years.  I reckon stealth technology
> is still effective against bush-league opponents, but I would be very
> surprised if anybody could fly into Chinese airspace without being
> picked up.

Picked up is one thing. Shoot down? If you don't have any
interceptors, you will be relying on SAM missiles. Vietnam-era designs
like the S-75 use shorter frequencies for technical reasons I don't
recall, and so will not be effective against stealth planes. Bistatic
installations cannot be easily mobile, which is where cruise missiles
come in.

Stealth is not about perfect undetectability, but about reducing the
effectiveness of long-range radar based weapons, which for physics
reasons have to use high frequencies. The engagement range of a
stealth aircraft is longer than that of its opponent, providing ample
opportunities to destroy the opponent first. You cannot fit an L band
antenna onto a SAM missile without serious size problems.

Sincerely,
Watson Ladd