Record-Breaking Experiment Quantum Entangles Two Atoms 20 Miles Apart

Perhaps the first step towards the development of a quantum internet has been taken by researchers who quantum entangled two stationary atoms over 20 kilometres of fibre optic wire.

Two stationary rubidium atoms have been quantum entangles across a record distance. Image: ezphoto

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.


The coupling of two particles so that modifying one instantly modifies the other is known as quantum entanglement. Moreover, one may automatically determine the state of the other particle by measuring the state of one.


The study authors explain how they entangled two atoms kept in different buildings on the LMU campus, around 700 metres (2,300 ft) apart, in a paper published in the journal Nature. The 33 kilometres (20 miles) of fibre optic cable that connected the two locations was routed through many coils.


After the two atoms were excited by a laser pulse, each of them released a photon. Most importantly, this mechanism causes the atom's spin to become quantum entangled with the photon's polarisation upon emission.


Because photons with wavelengths inside the visible light range of the electromagnetic spectrum often only travel a few kilometres down the cable before disappearing, previous attempts to transport such particles through fibre optics have failed.


 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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