Recently I have been getting a lot of questions from colleagues and friends about what is so special about quantum entanglement and how it works, so I thought it would be a good idea to share a few short details on this topic here.
Entanglement is one of the key powerful components of quantum mechanics that is not present in classical mechanics. It is also one of the main tools we rely on for the advantage of quantum communication.
Entanglement is a phenomenon where you can have multiple particles (or qubits -short for quantum bits-) in a state of correlation with each other. The state of each particle cannot be described independently of the state of the other ones. The correlation is maintained between these qubits even if you physically separate them in 2 distant locations. The spookiness of this correlation is that when physically separated, if you observe the state of the of the particles, it changes immediately, since as we mentioned they are correlated. This is known as non-locality. And It is the concept that puzzled Einstein so much that he described it as a “spooky action at a distance“.
So to illustrate, assume we have 2 qubits. We create them in a lab in a manner such that they are entangled. Now we separate them and send one of the qubits to India and the other to France. If someone performs an action (i.e a measurement) on the qubit in India, the state of the part that is in France will immediately change.
Here is a great illustration of this, that I borrowed from Quantum-magazine.
We can see here both are in some “unknown” state, but as soon as perform action on one part and see it is red, the other part of the system becomes blue. So it is like transferring the answer to a yes/no question.
So since the action happens immediately, does that mean we can transfer information faster than light?
Quantum entanglement is not Faster-Than-Light (FTL)
Despite the fact that the effect happens immediately, the truth is that after the non-local “information transfer” via quantum correlation, we still need to “observe” what happened to our qubit and measure the state of the system to see if it is blue or red. The speed of that measurement we perform locally (in our system) is still limited by the speed of light.
Therefore, Quantum nonlocality does not allow for FTL communication and it does not break the universal speed limit. In fact, despite the non-locality between distant systems, quantum theory is still local per single system in the strict sense defined by special relativity.
Quantum entanglement does not need a physical medium
Another thing that puzzled people I discussed with, is how the physically distant qubits tell each other to change. Do we need to build some fiber-optic medium between them? The answer is No, you can, but you don’t have to.
In theory, quantum entanglement does allow medium-free communication!
While many experiments still used physical medium because our qubits are very fragile, it is actually possible (as you may have guessed from my explanation) to perform communication between the 2 entangled systems without any physical medium linking them. Only the entanglement and local measurements suffice to transmit information.
If you are curious about this: you may want to read about “Remote quantum entanglement”. Here are 2 interesting papers on this topic:
I hope this small article was useful. As always do not hesitate to correct me if you think there are any mistakes or if you have any questions.