‘Spooky Distance Effect' In The Physics Building: Researchers From Saarbrücken Are Developing Building Blocks For Quantum Repeaters
Physicists of the University of the Saarland have managed to interlock an atom with a light quantum (photon) in the so-called telecom wavelength range. This allows quantum information to be transported over long distances using low-loss photons. The results have already received much attention from quantum researchers and have now been published in Nature Communications.
The communication with quantum states is extremely safe, since any interception would disturb the transmission signal and therefore it is always detected. For the same reason, it is difficult to transport the information over long distances: the signal that becomes weaker with distance can not simply be received and amplified by signal amplifiers, so-called repeater stations, as is the case in classical telecommunications.
For this another principle must be used: the quantum repeater. First, a so-called quantum entanglement over shorter distances is generated and then spread over longer and longer distances. A quantum entanglement between two particles exists if one can accurately describe the mutual state of the two particles with a fixed relationship, but each particle is measured by itself but found in a random, unpredictable state. One way to realize this is to interleave single atoms and single photons (light quanta). This is done in the laboratories of Prof. Jürgen Eschner using individual calcium atoms controlled by laser impulses in an ion trap ( https://idw-online.de/de/news614634). However, for the wavelength at which this works well (854 nanometers), there are no glass fibers for low loss transmission over long distances; instead one would like to send the photons in the so-called telecom wavelength range (1300 to 1550 nanometers). The technology for the conversion of photons into this area, the quantum frequency converter, was developed by Prof. Christoph Becher and his research group ( https://idw-online.de/de/news499868 ).
Together, both groups have now demonstrated that the telecom photon after the frequency conversion is still entangled with the atom that emitted the original photon. In addition, the researchers have shown that the high quality of the entanglement remains virtually unchanged. Fascinating here is that the common quantum state of microscopic particles (single atom and single telecom photon) extends over several floors of the Saarbrücken physics building. "There's nothing in the way of an entanglement over a distance of 20 kilometers," comments Matthias Bock, a graduate student in quantum technology and first author of the current study. The results are an important step to integrate quantum technology into conventional telecommunications;
Example of quantum entanglement:
For quantum bits - for example, atoms with two energy states for electrons or photons with two oscillation directions - the individual state can be illustrated by a point on a spherical surface. In a measurement, a single quantum bit is thus (unpredictably) found anywhere on the surface of the sphere. The other, which is entangled with him, but then always found in the opposite point on the ball. This correlation can also exist over long distances. This phenomenon, referred to by Einstein as "spooky action at a distance," is one of the hard-to-imagine peculiarities of quantum physics, but has been confirmed experimentally in many cases.
Source: The University of the Saarland