[Cryptography] Spooky quantum radar at a distance

[John Denker]

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.

The article claims to be "news", but I'm not sure why.

  g) The idea of quantum illumination goes back to 2008.
   Lockheed has a patent on "quantum radar" from 2008.
   There are entire textbooks on the topic of quantum radar.

  h) If this were a real breakthrough, it would be highly classified.
   We would not be reading about on the CETG or SCMP web sites.

> China's first "single-photon quantum radar system" 

  i) Mentioning single photons in this context is misleading at best.
   Quantum illumination systems use (or should use) coherent states,
   aka Glauber states.  The Glauber-state basis is /incompatible/ with
   the photon-number basis.  They are incompatible observables, in the
   Heisenberg sense.   A Glauber state looks a whole lot more like a
   classical AC voltage than like a single photon.

  j) Counting photons would be insane.  The photon number operator is
   /quadratic/ in field strength.  In second-quantized language, we can
   write it as (a†a).  For a small signal, the photon number is small
   squared.  Since the dawn of time, every radar ever built has instead
   measured the voltage, which is /linear/ in field strength, namely
   (a† + a).

  k) There is no fundamental dividing line between quantum noise and
   classical thermal noise.  They are two limiting cases of the same
   fundamental physics.  The relevant scale-factor is Boltzmann's constant
   divided by Planck's constant, which comes out to 21 GHz per Kelvin.
   At the frequencies we are talking about (X-band and below) and at the
   temperatures we are talking about (300 kelvin or so), it's essentially
   all thermal.  Calling it "quantum" radar is mostly marketing hype.
   A careful classical analysis of the apparatus would give essentially
   the same answer.

  *) Et cetera.  I could go on, but why bother?  If you want to know
   about the interesting stuff going on in the radar world, this article
   is not a good place to start.