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In last week Nature Physics there is nice experiment about controlling in the future if photons in the past are entangled or not:

Victor chooses later (QRNG) if Alice-Bob photons are correlated (left) or not (right):

so obtaining |R>|R> means that more probably Victor has chosen entanglement, |R>|L> separation - there is nonzero mutual information, so once again I don't see a reason it couldn't be used to send information?
Here is good informative article with link to the paper: http://arstechnica.c...-beforehand.ars

ps. If someone is anxious about the "conflict" of fundamental time/CPT symmetry with our 2nd law-based intuition, it should be educative to look at very simple model: Kac ring - on a ring there are black and white balls which mutually shift one position each step. There are also some marked positions and when a ball goes through it, this ball switches color.
Using natural statistical assumption ("Stoßzahlansatz"): that if there is p such markings (proportionally), p of both black and white balls will change the color this step, we can easily prove that it should leads to equal number of black and white balls - maximizing the entropy ...
... from the other side, after two complete rotations all balls have to return to the initial color - from 'all balls white' fully ordered state, it would return back to it ... so the entropy would first increase to maximum and then symmetrically decrease back to minimum.
Here is a good paper with simulations about it: http://www.maths.usy...ts/kac-ring.pdf
The lesson is that when on time/CPT symmetric fundamental physics we "prove" e.g. Boltzmann H theorem that entropy always grows ... we could take time symmetry transformation of this system and use the same "proof" to get that entropy always grows in backward direction - contradiction.
The problem with such "profs" is that they always contain some very subtle uniformity assumption - generally called Stoßzahlansatz. If underlying physics is time/CPT symmetric, we just cannot be sure that entropy will always grow - like for Kac ring and maybe our universe also ...

I apology for digging this thread up, but finally there is practical implementation and it beats modern state of arts methods in many application. It can be seen as greatly improved convolutional code-like concept – for example no longer using convolution, but carefully designed extremely fast operation allowing to work on much larger states instead. Other main improvements are using bidirectional decoding and heap (logarithmic complexity) instead of stubbornly used stack (linear complexity). For simplicity it will be called Correction Trees (CT).
The most important improvement is that it can handle larger channel damage for the same rate. Adding redundancy to double (rate ½) or triple (rate 1/3) the message size theoretically should allow to completely repair up to correspondingly 11% or 17.4% damaged bits for Binary Symmetric Channel (each bit has independently this probability to be flipped). Unfortunately, this Shannon limit is rather unreachable - in practice we can only reduce Bit Error Rate (BER) if noise is significantly lower than this limit. Turbo Codes (TC) and Low Density Parity Check (LDPC) are nowadays seen as teh best methods – here is comparison of some their implementations with CT approach – output BER to input noise level:



We can see that CT still repairs when the others has given up – making it perfect for extreme application like far space or underwater communication. Unfortunately repairing such extreme noises requires extreme resources – software implementation on modern PC decodes a few hundreds bytes per second for extreme noises. Additionally, using more resources the correction capability can be further improved (lowering line in the figure above).
From the other side, CT encoding is extremely fast and correction for low noises is practically for free – like up to 5-6% for rate ½. In comparison, TC correction always requires a lot of calculation, while LDPC additionally requires also a lot of work for encoding only.
So in opposite to them, CT is just perfect for e.g. hard discs – everyday work uses low noise, so using CT would make it extremely cheap. From the other hand, if it is really badly damaged, there is still correction possible but it becomes costly. Such correction itself could be also made outside, allowing for further improvement of correction capabilities.

Paper: http://arxiv.org/abs/1204.5317
Implementation: https://indect-proje...rrection-trees/
Simulator: http://demonstration...orrectionTrees/

swansont, on 20 April 2012 - 03:09 PM, said:

You could also just absorb these photons/electrons with their angular momentum - I think it's doable experiment:

Shoot with polarized electrons at object floating on surface of conductive liquid - it would absorb polarized electrons, then release unpolarized ones - so the difference of average angular momentum should make it rotate.

I think you couldn't rotate it this way, but maybe I'm wrong ?

My sarcasm was only about "universal answer" when physicists don't understand something: "it's quantum.".
While quantum mechanism is kind of extension of classical one with the wave nature of particles and in first approximation (h=0) of QM you still get classical mechanics - quantum concepts are not unreachable for our minds, if only we stop basing on such imaginary limit of our understanding.
We shouldn't just "shut up and calculate", but still try to understand e.g. dynamics behind wavefunction collapse, field configurations behind particles ... and many other important fields ignored because of "it's quantum" universal answer.
Have a good weekend.

Angular momentum conservation says that if we would take a lot of "tiny angular momentums", we should be able to rotate a macroscopic object - and it works for photons. So shouldn't it also work for electrons?
If electron's angular momentum doesn't correspond to some kind of rotational motion, so ... what does it even mean? It's not a real angular momentum, but only some kind of quantum-transcendental one?

Quantum rotation operator says precisely what does e.g. electron's spin means - that while rotating the system by alpha angle, quantum phase changes by spin*alpha - it is also definition of Conley topological charge.
Accordingly to it, situation near +1/2 (left) and -1/2 (right) spin looks something like here:

This picture also solves another transcendental "quantum mystery" - that some objects rotated by 2pi becomes something different ... it can be repaired using some covering groups ... or just using field with some symmetry: saying that phase and minus phase is the same as on this picture.
And something very similar we have while explaining that magnetic flux going through superconducting ring is quantized: because the phase around has to make integer multiplicity of 2pi:

... or understanding quants of magnetic flux in superconductor: fluxons/Abrikosov vortices - configurations stable because of topology - topological solitons.
Their magnetic field is because of topological singularity of quantum phase - like for electron.

I'm a bit confused - so in the first statement here you say that electron has angular momentum: is spinning, while in the second that it is not?
It's a part of quantum transcendentalism? Like it is both wave and particle, but isn't?