Quantum communication - Part 2

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4 Jul 2022

What are the technological barriers to achieve quantum communication?

China is now the world leader in quantum communication with quantum key distribution technology. They are able to distribute pairs of entangled photons upto 7400 km apart and use the pairs as cryptographic keys to open encrypted messages, it facilitates hack-free communication as any eavesdropping of the light beams would collapse the entangled photons and alert security management.

So what are the cool things they have done in quantum communication technology. Well, making the entangled photons was easy, but the most difficult challenge of the project was to focus the beam of entangled photons onto the tiny target of the receiver on earth about 2cm in diameter from the satellite 600km above ground in the sky. That is a very difficult task to achieve in the field.

The other challenge was to distinguish the entangled photons from background noise of trillions upon trillions of other photons from the sun, stars, moon and city lights in the sky. These background noises need to be eliminated so that only the entangled photons are received and captured from the communication satellite. According to China’s leading quantum physicist Jian Wei Pan (see below photo) they achieved this by two methods. Firstly, by setting the receiver to look at a specific wavelength of the entangled photons but ignore others. Secondly, by setting the receiver to observe only at specific time frame that taken the entangled photons to travel to ground from the satellite but ignore other photons arrive at the receiver in other times frames.

Once they received the signals on the ground they still have many potential background noises, so they also need to filter other potential remaining noises before obtaining the cleaned distributed quantum keys.

Faster than light speed communication

Can this technology get a breakthrough and achieve faster than light communication? Jian Wei Pan said confidently no to the question. The technology will still be bounded by the limit of physics in our universe and cannot break the light speed barrier by any other means and propbably never will.