Physicists exchanged quantum entanglement at a distance of 100 kilometers.

Chinese scientists were able to make an exchange of quantum entanglement between photons from quantum-entangled pairs separated by an optical fiber longer than 100 kilometers. This result exceeds by distance all previous similar attempts and expands the possibilities of quantum teleportation with the exchange of entanglement to interurban scales. 

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Quantum entanglement - the ability of two photons to maintain an interrelated quantum state. When the quantum state of one of the photons changes, the state of the second one changes instantly. It is noteworthy that while maintaining the coherent coupling between two photons, entanglement can be observed for any distance between them. This property scientists proposed to use for the mechanisms of quantum teleportation - the instantaneous transmission of quantum information at a distance. In order to avoid decoherence of photons and loss of entangled state, the concept of a quantum repeater was proposed. This concept is based on the use of entanglement swapping between photons from two independent quantum-entangled pairs. This leads to the fact that information about the quantum state can be transmitted even between two photons, which are at a great distance from each other and were not initially entangled with each other.

The fundamental possibility of implementing quantum teleportation with the help of such an exchange was shown for both satellite and fiber-optic transmission of photons at a distance of about 100 kilometers. However, all these mechanisms were implemented only for the exchange between photon pairs, which were obtained using a single source. In order to truly experimentally confirm the interchange of entanglement, at least two independent sources of entangled photons are needed and the absence of a cause-and-effect relationship between events that lead to a change in the photon quantum state.

In their new study, Chinese scientists used two sources of entangled photons with a frequency of 1 Gigahertz, and conducted a field test on the exchange in a fiber optic cable 103 kilometers long. 77 kilometers of this cable were inside the laboratory, a 25-kilometer section ran underground and another one kilometer of cable was in open areas, being exposed to external noise.

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The experiment was carried out in such a way that sources and signal detectors were installed at three points. Two independent sources at points A and B generated an optical signal with a frequency of 1 GHz. Some of the resulting photons remained in the spiral optical fiber near the source, and the other part was sent without loss of coherence to point C (yellow lines in the diagram). After that, using short laser pulses that were sent from point C to points A and B, the signals were synchronized (the purple lines on the diagram), linking the states of those photons that remained near the source.

As a result of the experiment, scientists managed to exchange an entangled state between photons from points A and B. The signal loss during transmission was no more than 16 decibels, which is approximately 20 decibels higher than previous experiments. Thus, scientists have shown that combining sections of a spiral and branched optical cable, it is possible to create systems of quantum teleportation with the exchange of entanglement, in which the points are spaced 100 kilometers apart.

The interchange of entanglement is an extremely important task for creating quantum repeaters and increasing the length of quantum teleportation. Recently, scientists were able to create a system in which photons from two independent tangled pairs can also exchange an orbital angular momentum, which dramatically increased the amount of information that can be transmitted. And the maximum distance of quantum teleportation without the exchange of entanglement already exceeds one thousand kilometers.