Comments about the article in Nature: Quantum-teleportation experiments turn 20

Following is a discussion about this News & Views in Nature Vol 552 7 December 2017, by Nicolas Gisin
To read the article select: https://www.nature.com/articles/d41586-017-07689-5

• The text in italics is copied from the article
  
• Immediate followed by some comments
  

In the last paragraph I explain my own opinion.

Introduction

The article starts with the following text:

Who has never dreamed of undergoing teleportation?

How can you answer this question when you do not know what teleportation is. The article should start with a clear definition. Sorry it does not.

Especially when stuck in a traffic jam.

What has that to do with teleportation?

To get disembodied here and reconstituted at a distant location sounds both marvellous and impossible.

Ofcourse you don't want to be in a traffic jam and if you are, you want to be somewhere else. But that is physical impossible.

But is it really impossible.

Yes

True, a body made out of matter cannot merely disappear here and reappear there without travelling the intervening distance. But an object is not just matter, it is also a structure - substance and form as Aistotle taught us long ago; particles and quantum states as physicists would say today.

Replace the word object by human being and you can see that this sentence does not make sense. Anyway Aristotle has nothing to do with teleportation.

Twenty years ago, Boschi et al. and Bouwmeester et al. used this idea to perform the first quantum-teleportation experiments, which led to major advances in quantum-information science.

For more detail about the experiment "Experimental quantum teleportation" by Bouwmeester read this:
https://web.physics.ucsb.edu/~quopt/exp.pdf
What you can see is that the article heavily depents on entanglement.
One sentence in this article reads:
"However, according to Heisenberg's uncertainty relation, one cannot determine the state of a single quantum system by measurement."
How can you prove such a claim ?

In 1993, etc. Entanglement is a phenomenon in which two or more quantum particles share a common state, such that each particle cannot be described independently.

The concept of Entanglement is based on certain experiments in which two particles are involved. In these experiments identical parameters of each particle are measured. The issue is that the results of these measurements are highly correlated in the sense that, if one particle measures a "1" the other one will measure a "0". That is all.
See also: Reflection 1 - Bell-state Entanglement

Non-locality refers to the observation that spatially separated quantum particles behave in ways that defy our intuition about space and time.

This requires a much longer explanation.

The quantum state would therefore disappear from the sender and reappear at the receiver.

It is logical a transfer operation.

Figure 1 | Quantum teleportation Twenty years ago, Boschi et al and Bouwmeester et al showed that the quantum state of a photon can be teleported from a sender to a distant receiver. Did they really show this, demonstrate this in e real experiment or is this athought experiment? One of the entangled photons is given to the receiver — in principle, in advance of the quantum-teleportation process. Why do they write "in principle". This smells that this is a thought experiment. The sender then prepares a photon in an unknown quantum state and combines this photon with the second entangled photon in a device called a Bell-state analyser. This seems rather simple but the technical implications can be exceptional. See also Reflection 2 - Bell-state analyser

At page 43 we can read:

Fully understanding quantum teleportation is tricky and the concept initially led to some confusion.

If you want to explain quantum teleportation you have to start with a clear demonstration.

The article ends with the following sentence:

Twenty years on from its first experimental demonstrations, quantum teleportation is a tool that will allow the highly successful community of physicists, engineers, computer scientists and mathematicians to work together to develop the next generation of quantum-communication systems and quantum computers.

Sorry to say but this is a dream.

Reflection 1 - Quantum teleportation

IMO the article gives a to much optimistic idea about the present state of art.
Yes particles can be entangled. To demonstrate this you have to perform 1000 identical experiments. The results will show that the particles are correlated, but this correlation in the measurements does not have any physical implication. The cause of the correlation is in the reaction that created the two photons.
The problem is can you can not use such a correlation (as far as I know) to perform any other usefull experiment. You cannot use this to teleport a human being from one place to an other. You also can not use this to send usefull information between to parties.
In principle you can send two entangled messages between two parties:

One will receive the message: and the other one:

"00010010001101001100011110111110" "11101101110010110011100001000001"

but what does each message mean? (except that they are correlated). Each message is completely random. In hexadecimal notation the first message reads: "1234c7be".

Reflection 2 - Bell-state analyser

        BSA 
      0--> 0    
    .     .   .    
  .      .       .
0       .          1
   .   .         .
      0---EP--1

        BSA 
      0--> 1    
    .     .   .    
  .      .       .
0       .          1
       .         .
      1---EP--0

        BSA 
      1--> 1    
    .     .   .    
  .      .       .
1       .          0
       .         .
      0---EP--1

        BSA 
      1--> 0    
    .     .   .    
  .      .       .
1       .          0
       .         .
      1---EP--0

• The middle line shows at the left side the state of the photon near the Sender to be teleported. The right side shows the the state of the photon received with the Receiver. Investgating all 4 frames the results shows that they are each other complement. A simple inverse operation will make both the same, which is the object of this teleportation experiment.
• The bottom line shows the two entangled photon. Because there are two possible input conditions at the Sender and two possible states of the entangled photons, this explains the 4 frames.
• The bottom line shows the Bell State Analyser. The left state of the BSA is a copy of the state of the sender. The right state is an exclusive-or operation of the left state of the BSA and the left state of the Entangled Photon.
• The final state at the receiver is an exclusive-or of the right state of the BSA and the right state of the entangled photon.

• IMO the biggest problem to send one of the entangled photons towards the Bell-state analyser and the other one to the Receiver. Part of the issue is how do you test that the two photons stay entangled i.e. are correlated.
• A different problem is the operation performed in the Bell-state analyser and inside the Receiver. In both cases I assume this is an exclusive-or operation because such an operation gives the desired result. The issue to perform these operations in reality because they have to be performed exactly at the same moment when the photons arrive. That is tricky.

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