"Spooky action at a distance" harnessed for telecoms

Physicists have succeeded in mastering a technique, which could allow transmission of quantum information over long distance.

Professor Jürgen Eschner and his team at Saarland University in Saarbrücken, Germany, have entangled a single atom with a single photon in the telecom wavelength range. This constitutes a basic building block for transmission of quantum information over long distance with low loss.

“One of the fascinating aspects of the work is that the entangled quantum state of the two microscopic particles extends over several floors of the physics building of the university,” said Matthias Bock, PhD student in quantum technologies and first author of the study published in Nature Communications.

“This paves the way for entanglement over 20 kilometers and more. The results are an important step towards integrating quantum technologies into conventional telecommunications.”

Utilising quantum states offers the potential of high security communications, because eavesdropping attempts disturb the signal and would therefore be detected. For the same reason, though, long-distance transmission of that information is difficult. In classical telecommunication, the weakening of the signal is counteracted by measuring, amplifying and re-sending it at so-called ‘repeater stations’, but this turns out to be as detrimental to the quantum information as an eavesdropper.

To overcome this the team used a different principle: the quantum repeater. Quantum entanglement is first established over short distance and then propagated to longer separations. Quantum entanglement between two particles means that their common state is precisely defined, although when the individual states of the particles is measured, the results are random and unpredictable. A possible solution to this is to entangle a single atom with a photon that it emits. This is what Professor Eschner has managed to achieve using single calcium atoms in an ion trap controlled by laser pulses.

To really be able to use this as a communications tool however the phenomena has to be harnessed at distance. To do this the photons need to be transmitted in one of the so-called telecom bands (1300 – 1560 nanometers). The technology for converting the photons into this regime, the quantum frequency converter, has been developed by Professor Christoph Becher and his research group. Together, the two groups have now demonstrated that after quantum frequency conversion, the telecom photon is still entangled with the atom that emitted the original photon, and that the high quality of the entanglement is maintained.