[Cryptography] Spooky quantum radar at a distance

[Jerry Leichter]

> 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.)

The article seems to hint that they are doing something of this general sort, but it's really impossible to tell.  It's full of all kinds of buzzwords and hints, but what exactly it's doing you can't determine.

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.  This is based first off on a misunderstanding (in the press article) about how "spooky action at a distance" actually works.  It never actually transfers information - QM doesn't violate relativity theory, which says information can't be transferred faster than the speed of light. The "spooky action" is really spooky - what gets transferred is correlations between the two ends.  Yes, a photon that makes it to the stealth airplane interacts with it *somehow*.  Traditional radar learns something if the interaction reflects the photon back towards the source.  If it's reflected off in some other direction (what the shape of a stealth aircraft is all about) or absorbed (what the coatings do), radar can't distinguish that from "the photon flies by the plane and flies into space".  The article would seem to hint that somehow measuring the stay-at-home photon gives you information about the difference among "bounces in another direction", "is absorbed", and "flies into space".  With exactly that description, it sounds impossible; correlation between the two photons is *not* communications.  Add something else - perhaps a bunch of sensors with a broad view of the entire volume of space in which the plane might be, and combine that with the "stay at home" photon (similar to what happens when you use the reference beam to reconstruct a hologram) ... that at least sounds physically plausible.  How it might work ... who knows.

Based just on this article, I'd say bullshit.  If there's really something there, we'll hear more about it.  Stealth technology (which, ironically, Americans developed based on a paper by a Russian mathematician, whose work wasn't recognized in his own country, possibly because the computation required were well beyond their capabilities at the time) probably stayed secret longer than pretty much any advance based on physical principles; but even it came out in the end.  If there's something here ... it'll come out, too.

                                                        -- Jerry