Torquemada 1420 wrote:
A5D5E5 wrote:
I'm not 6roucho (nor even close), but I'll have a go at explaining this.
Quantum entanglement creates a pair of particles that "know" exactly what the other is doing - even when they are separated (in this case by a very long distance). This knowledge exchange happens instantaneously (seemingly - certainly much, much more quickly than the speed of light) [but as information can't be transmitted by this mechanism, it doesn't break the rules around relativity and the speed of light which strictly apply to information transmission].
I think what this new announcement is about is using entangled particles as a way to detect if somebody without the appropriate "key" is trying to access the encrypted message. There is a process called decoherence where particles "lose" their quantum mechanical properties (opening the box and looking at the cat for example). This seems to be the way that the encrypted information is destroyed if an unauthorised person tries to read it.
The big deal is the sheer distance over which the particles are separated - an engineering masterpiece by the looks of it.
As to how that is all done - you will need to ask somebody with a nobel prize in physics.
Help. What is the new bit here? This is Einstein's "spooky actions at distance" from the '30s?
Yes.
The basic postulates of schrodinger flavoured non-relativistic quantum mechanics that connect the mathematical theory to observable reality include:
(i) all the information you can get about a quantum system is described mathematically in its wave function (this gives a complex number for every possible state the quantum system
could be in if observed),
(ii) that every observation of a quantum system corresponds to a mathematical object called an operator (this is something that maps wave-functions to other wave-functions), and
(iii) the chance you
actually observe something is proportional the amplitude of complex number that possibility was given by wave function.
(iv) once you observed something the wave function is "collapsed" and now assigns 100% probability to the observation (technically any possible distinct observable value is an eigenvalue of the observation operator and the wave function collapses to become the associated unique eigenfunction within the Hilbert space you've constructed to understand the quantum system mathematically).
Einstein (and Podolsky and Rosen in Physical Review vol. 47 1935) noticed in the standard maths of QM that it was
possible to have two connected quantum systems (called I and II) that get their joint wave-function, you let them separate so they are not interacting (basically no distance term exists in the equations), observe some stuff about the first system (which collapses the wave function), and for the collapsed wave function to specify that the other system is now in the eigenfunction state for some definite measurements (lets say for quantities P and Q). Basically, QM allowed observing system I to make the observation of system II definite:
EPR in 1935 wrote:
This makes the reality of P and Q depend on the process of measurement carried out in the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this.
Except that not only does reality permit this... you can actually demonstrate this mad brain bending shit! This effect is what we now call quantum entanglement.
My understanding is that the chinese quantum security mechanism doesn't make uncrackable security but rather it makes it impossible to read (i.e. observe and collapse its quantum state) without that observation being detected.