Quantum Teleportation

Anton Zeilinger

This year the Nobel Prize in Physics has been awarded to John Clauser, Alain Aspect and Anton Zeilinger, for their pioneering work in quantum communications.

• Clauser (with Stuart Freedman, who died in 2012) carried out the first experiment with entangled photons in 1972, which proved that the direction of Bell's inequality favors the Copenhagen interpretation of quantum mechanics, thus solving the EPR paradox (acronym for Einstein-Podolsky-Rosen), which tried to contradict this interpretation. This experiment was not complete, so others were carried out later, under increasingly strict conditions.
• Aspect performed the second experiment using faster components, which completed the previous experiment and again confirmed that Bell’s inequality favors the Copenhagen interpretation.
• Zeilinger has worked mainly on quantum teleportation, which is what we are going to talk about in this post.

First of all, a clarification: in these experiments matter is not teleported, not even photons. What is teleported is the quantum state of a photon, by exchanging it with that of another photon, more or less apart from the first. In other words, at the end of the process, two photons have exchanged their quantum states. Here is a brief explanation of Zeilinger's experiments:

1. We initially have three photons: A, B, and C. The quantum states of A and C are entangled, which means that they share a quantum property, either because that property has the same value for both photons, or because one has the opposite value to the other, although in principle we do not know which has which. In contrast, the quantum state of B has no relation with that of A and C. These entangled quantum states are called Bell states, because they are used to demonstrate the sign of Bell's inequality. On the other hand, a pair of photons with entangled quantum states makes a qubit of quantum information. (See this blog post).
2. The second part of the procedure consists in creating a quantum channel between photons A and B. Quantum information can be transmitted through quantum channels. In the experiment we are describing, what is done is to exchange the quantum states of the photons connected by the quantum channel. At the end of this phase of the process, B has the quantum state that A previously had, while A has now the quantum state previously belonging to B.
3. As A’s quantum state was initially entangled with C’s quantum state, at the end of the process B’s quantum state is entangled with that of C. In other words, at the beginning C’s state was entangled with A’s state; at the end it is entangled with B’s state, without C having done anything. Therefore, a teleportation of one qubit has taken place from A’s position to B’s position.

Zeilinger and his team have performed the following experiments:

• In 1998 they exchanged the quantum states of two photons separated by a meter.
• In 2004 they did it between two photons located on both sides of the Danube, 600 meters apart.
• In 2012 the teleportation was carried out between two photons located in the Canary Islands of La Palma and Tenerife, at a distance of 143 kilometers.
• In 2017, a Chinese team used a similar procedure to exchange the states of two photons, one on Earth, the other in a satellite in space, breaking the record for teleportation of quantum information over distance.

In any case, it is worth remembering once again that in these experiments matter is not transported, just information in the form of a quantum entanglement.

It is also interesting to note that Zeilinger declares himself a believer. He did it, for instance, in this interview that appeared in 2012 in the Spanish newspaper La Vanguardia.

The same post in Spanish

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Manuel Alfonseca