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 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

The following 4 frames describe the 4 possible outcomes of photon teleportation.
        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
Each frame consist of three lines. From a theoretical point of view all this looks rather simple. From a technical point of view this is very complex process.

Feedback

None


Created: 16 February 2018

Back to my home page Index
Back to Nature comments Nature Index