Ludwig Maximilian University (LMU) physicists have just broken the record for quantum entanglement by successfully connecting two rubidium atoms over a 33-kilometer (20-mile) fibre optic connection. The accomplishment is a significant step towards the creation of a quantum internet, which would enable instantaneous information transfer between network nodes.
When two particles are paired together in such a way that altering one instantly affects the other, this is known as quantum entanglement. Moreover, you can automatically determine the state of the other particle by measuring the state of one.
The study's authors, who published their findings in the journal Nature, detail how they managed to entangle two atoms that were kept in different buildings on the LMU campus, around 700 metres (2,300 ft) apart. The 33 kilometres (20 miles) of fibre optic cable that connected the two locations was routed through many coils.
The two atoms were excited by a laser pulse, which resulted in each of them emitting a photon. Most importantly, this mechanism causes the atom's spin to become quantum entangled with the photon's polarisation upon emission.
Previous attempts to transfer such particles down fibre optics have not succeeded because photons in the electromagnetic spectrum with wavelengths within visible light range often reach only a few kilometres along the cable before disappearing.
In order to raise the photons' wavelength from 780 to 1,517 nanometers—roughly equivalent to the telecom wavelength of 1,550 nanometers—the researchers employed "polarization-preserving quantum frequency conversion." This is the optimal frequency range for light transmission via fibre optics.
Because of this, the photons were able to make it through their record-breaking journey down the cable and be detected by a receiver. At this moment, the photons were measured jointly, entangled as a result. The two atoms eventually became entangled with one another as a result of this procedure because each photon was already entangled with the rubidium atom from which it was released.
The two atoms could function as "quantum memory" nodes in a larger communication network once they are entangled. The fact that fibre optic cables were used to accomplish this is significant because it opens the door to the potential of building such a network utilising already-existing telecom infrastructures.
Lead author Tim van Leent said in a statement, "The significance of our experiment is that we actually entangle two stationary particles, that is to say, atoms that function as quantum memories." "This opens up many more application possibilities, but it is much more difficult than entangling photons."
More specifically, "the experiment is an important step on the path to the quantum internet based on existing fibre optic infrastructure," according to co-author Harald Weinfurter.
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