• The text in italics is copied from that url
• Immediate followed by some comments
In the last paragraph I explain my own opinion.

### Introduction

The article starts with the following sentence.

### 1. Entanglement

A quantum system is described, at every instant, by a vector state which, according to the theory, represents the maximum amount of information that it is possible to have.
This sentence is very complex and requires a detailed explanation.
• First of all what is a quatum system? Is my PC also a quantum system?
• What is a state vector?
• What means information?
The problem is that my PC at each instant is a combination of zero's and one's which in total descibes the state as my PC. These zero's and one's are stored in bits of my PC. At the next instant the state changes as a result of logical operations on a sub set of the bits.
Is my PC a quantum system? Not in the sense of the subject of this article. My PC does not use the concept entanglement.
So what is entanglement. That is subject the article should start with.
To simplify discussion, let's take the example of the state of polarization of a photon and associate with it the vector state |45>
To understand this sentence the reader should at least know what means polarization. The Wikipedia document Polarization is too general (and rather complex).
For example in the case just referred to we know that if we were to apply a test for vertical polarization to the photon whose state is|45> , it would have a probability of 1/2 of passing and 1/2 of failing.
How do we know that the state is |45>? This sentence requires a better explanation.
This is an important distinction with the situation in classical mechanics: in classical physics, any system always possesses precise values for all of the observables that we can conceive, but in quantum physics, a single system will indeed possess some property, but, with reference to other properties, we can do no better than make probabilistic prediction about the results of possible measurements, of and when they are actually executed.
In classical physics you study the "state" of a sytem in global sense. In quantum physics you try to study the state of the individual particles.
To think that in a classical system all the observables (parameters) have precise values is a misnomer. Consider the temperature in an ocean.
In quantum mechanics to study the parameters of the individual particles you have to perform many identical tests to learn the expected values. The results of this test allow you to calculate the probabilistic predictions of the next test.
Now consider the following situation: two photons are emitted by a source S and are propagated in two opposite directions. At a certain instant, one of them can be found in the region A, to the right of the source and the other in the region B symmetric to A with respect to S
This "experiment" is not clear. What does it mean "can be found in the region A"
We can call the photon at the right 2 and assume that it possesses a vertical polarization. This can be indicated as the vector state |2,V>. Analogously, suppose that the photon on the left, indicated as 1, has a horizontal polarization, so that it is described by the vector state |1,H>
Why do they use the word: assume?. All of this is not clear.
See Reflection 1

### Reflection 1 - Experiment

In the above text an experiment with two photons is described. This experiment is not clear. Let me try to give my own interpretation.

Consider a source S which transmits two photons in opposite directions. The direction at the left is called A, towards the right is called B.
At both points A and B the photons are "measured" by means of a filter in the same vertical direction. There after the photon can be detected by means of a photon detector.
At detector A the outcome of each experiment can be 0 or 1.

• 0 means: there is no photon detected.
• 1 means: a vertical polarized photon is detected.
For both detector A and B the following combinations are possible:
1) 0,0 or 2) 1,0 or 3) 0,1 or 4) 1,1
1. Combination 0,0 means that neither side a photon is detected.
2. Combination 1,0 means that only at side A a verical photon is detected.
3. Combination 0,1 means that only at side b a verical photon is detected.
4. Combination 1,1 means that at both sides a verical photon is detected.
In practice many completely different experiments are possible with always show outcome (4). In these cases the source S emits two unpolorized photons.
Experiments where you always get result (1) are of no interest. What we want to study are experiments with outcome (2) or (3).

Fortunately there are experiments where each time when you perform the same experiment you either get result (2) or (3). That means you have a source S and each time at least one of the photons is vertical polarised.
The next thing you can do change the direction of both filters 90 degrees in horizontal direction and repeat the same experiment. The result is the same: you get either a (2) or a (3). The interpretation is different: one of the photons is vertical polarised.
Combining both results you can come the following conclusion: I have a source S which transmits two polarised photons with the following condition: When the left one is polarised in the vertical direction the right one is polarised in the horizontal direction and vise versa. That means there exist a clear correlation between the two photons. In other words the two photons are correlated.

There is nothing tricky in this conclusion.
In fact you can also make the following conclusion: When I only test side A I can predict with 100% propability what the possible outcome of side B would have been. Also here nothing tricky.

The above experiment can first be performed in a laboratory i.e. local. The same experiment can also be performed over much larger distances. The results are the same: there is a clear correlation between each outcome

### Reflection 2 - General

The overall impression is that this document is too technical, too mathematical.
Understanding physics starts with experiments. Generally speaking when you perform the same experiment many times the outcome should be the same. With experiments in which entanglement is involved the outcome is often or A or B with a 50 50 percent chance of either one. The more often you perform the experiment the more accurate you can describe each. The actual outcome of each individual experiment is completely random. This is the type of information the document should discuss.

The whole point of the experiments is that there is nothing spooky involved. Correlation is not spooky when you start from a common source.

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Created: 10 December 2016

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