Quantum communication satellite

[pranj5]

We all know about the launching of a entanglement based communication satellite by China. But, once a Prof. of Quantum physics has told me that it's even "theoretically impossible" as it will be violation of the "law of casualty" as the two points get information ahead of the speed of light to connect the two point. I want to know the truth behind.
On a recent newspaper article, I have seen that the Chinese authority has stated that the satellite has transmitted GB of data. I want to know whether it's possible or not as per the laws of quantum physics.

 

[PeterDonis]

once a Prof. of Quantum physics has told me that it's even "theoretically impossible" as it will be violation of the "law of casualty" as the two points get information ahead of the speed of light to connect the two point.

Quantum entanglement, which is what I assume is being used here, does not violate causality and does not transmit information faster than light.

 

[pranj5]

As far as I can understand, entanglement means some spooky interaction between two particles, here this photon. If Entanglement has been used, why the data isn't transmitted at a rate faster than light?

 

[DennisN]

As far as I can understand, entanglement means some spooky interaction between two particles, here this photon.

No (not necessarily). The term "spooky interaction" is originally from a quote by Einstein. Entanglement means that measurements of two particles can be strongly correlated even when the measurements are done at two distant places.

 

[DennisN]

@pranj5, if you are interested in this, you can watch this clip, where the quantum physicist Anton Zeilinger describes entanglement (at 1:30 into the clip and onwards) and a variety of experiments/applications:
Anton Zeilinger - Quantum Information and Entanglement

 

[pranj5]

Entanglement simply means if we affect one particle, that too can affect the other particle. Right? And without that, no communication can be done. In this case, data has been transmitted from point A to point B without any kind of electromagnetic signalling. In that case, why the data transfer isn't done above the speed of light?
As far as I know, the two points are Vienna and Beijing and as the main reason behind this experiment is to find out a hackproof way of communication, therefore the time needed to transfer data from Vienna and Beijing isn't measured here. The satellite has sent two beams of entangled photons to the two points and the data has been transferred by affecting the photons on one point and measuring the change in the entangled photons in the other point. I am pretty sure that data transfer has been done at rate faster than the speed of light and it has been done in a time period much less that what light will take to travel from Vienna to Beijing.
Actually, I am more interested in how this can be used to transmit data.

 

[DennisN]

@pranj5, it would be a good idea to provide a source for these kinds of things.

I had a look on the net and found this article in PhysicsWorld:
http://physicsworld.com/cws/article/news/2016/aug/16/china-launches-world-s-first-quantum-science-satellite

According to the article the satellite will test quantum encryption:

Quantum encryption

The main goals of QUESS will be to demonstrate quantum key distribution (QKD) between the satellite and two stations on the ground – the Nanshan 25 m telescope at the Xinjiang Astronomical Observatory in western China and the Xinglong Observatory in Yanshan, around 200 km south of Beijing. A quantum key is a string of ones and zeros, representing the quantum states of particles. These can be used to encode and decode messages, which would be secure from eavesdroppers.

Quantum encryption is described in the clip I provided above. And for these things, there is no communication taking place faster than the speed of light.

 

[PeterDonis]

If Entanglement has been used, why the data isn't transmitted at a rate faster than light?

Because the correlations between measurement results due to entanglement do not convey any information until the knowledge of both results is brought to the same place, and that takes place at the speed of light or slower. This is thoroughly discussed in the literature on entanglement, not to mention in many threads in this forum.

 

[PeterDonis]

According to the article the satellite will test quantum encryption:

Quantum encryption uses entanglement, so asking about what is possible using entanglement is relevant.

 

[PeterDonis]

the data has been transferred by affecting the photons on one point and measuring the change in the entangled photons in the other point.

That's only part of the process. The other part is transmitting, by ordinary classical channels, the measurement results on the photons at the first point, to the receiver at the second point. Until those results are received, no information can be extracted from the measurement results on the entangled photons at the second point. That is why entanglement can't be used to transmit information faster than light.

 

[pranj5]

That's only part of the process. The other part is transmitting, by ordinary classical channels, the measurement results on the photons at the first point, to the receiver at the second point. Until those results are received, no information can be extracted from the measurement results on the entangled photons at the second point. That is why entanglement can't be used to transmit information faster than light.

In that case, this communication can't be hackproof as the classical channel can be hacked and the measurement results can be known to a third party.

 

[Nugatory]

In that case, this communication can't be hackproof as the classical channel can be hacked and the measurement results can be known to a third party.

The way the quantum encryption protocols work, knowing the information sent by the classical channel is not sufficient to read the message. There are a number of different protocols, but here's an example of how a simple one might work:

- Alice creates N entangled photon pairs and sends one member of each pair to Bob.
- Alice and Bob each measure their photons. Because of entanglement, they will both get the same results (although they have no way of controlling what that result is, which is why there's no way of sending any information this way).
- Although they haven't communicated any information, they now are both in possession of the same N-bit random number.
- Alice XORs this random number with the N-bit message she wants to send, and transmits that across the classical channel to Bob.

You can google for "Quantum key distribution" for more information, including the more elaborate and realistic protocols that are used in real applications.

 

[Nugatory]

Older thread: https://www.physicsforums.com/threads/china-launches-quantum-enabled-satellite-micius.882166/

 

[Ken G]

They also succeeded in teleporting a quantum state of a photon. This is a remarkable process, especially since it raises the important question of whether or not a quantum state is actually something physical that can actually be teleported in the common sense!