Comments about the Nature | News: China's quantum satellite clears major hurdle on way to ultrasecure communications

Following is a discussion about this News & Views in Nature Vol 546 15 June 2016, "Probe sends entangled photons — which could underpin quantum-based data encryption — over unprecedented distance". by Davide Castelvecchi
To read the article select: https://www.nature.com/news/china-s-quantum-satellite-clears-major-hurdle-on-way-to-ultrasecure-communications-1.22142

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

In the last paragraph I explain my own opinion.

Introduction

Researchers report in Science that, by beaming photons between the satellite and two distant ground stations, they have shown that particles can remain in a linked quantum state at a record-breaking distance of more than 1,200 kilometres.

This is an excellent result.
Unfortunate the article does not mention how many tests are performed and what the success rate is

Quantum communication is secure because any interference is detectable.

That may be true. The problem is how. There is a difference if you send a message back to the receiver or over a large distance from A to B.

Two parties can exchange secret messages by sharing an encryption key encoded in the properties of entangled particles; any eavesdropper would affect the entanglement and so be detected.

How is this encoding done. There is a big difference to send a random key from A to B than a predefined key. The second is much more difficult.

Bell test

In theory, entangled particles should remain linked at any separation. That can be checked using a classic experiment called a Bell test.

Such a theory is wishfull thinking.

For the Bell test, the beam was split to generate pairs of photons that share a common quantum state, in this case related to polarization. Once the particles arrived, the team used the Bell test to confirm that they were still entangled.

The details about how the Bell test over large distances should be discussed

Next-generation satellite

Reflection

The result of this experiment is ofcourse very clever. The problem is to what extend a defined physical state (a set of classical bits) can be translated into a set of entangled bits.

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