A new quantum entanglement record has just been set by physicists at Ludwig Maximilian University (LMU) who successfully connected two rubidium atoms across 33 kilometers (20 miles) of fiber optic cable. The achievement represents a major milestone in the quest toward a quantum internet, which would allow for the instantaneous transmission of information between nodes in a network.

Quantum entanglement refers to the pairing of two particles in such a way that changing one instantly alters the other. Furthermore, measuring the state of one particle automatically tells you about the state of the other.

Writing in the journal Nature, the study authors describe how they entangled two atoms housed in separate buildings on the LMU campus, roughly 700 meters (2,300 feet) apart. The two locations were connected by a fiber optic cable that ran through numerous coils and measured 33 kilometers (20 miles) in length.

A laser pulse was used to excite the two atoms, causing each to emit a photon. Crucially, this process results in the spin of the atom becoming quantum entangled with the polarization of the emitted photon.

Previous attempts to transmit such particles along fiber optics have failed because photons with a wavelength that falls within the visible light range of the electromagnetic spectrum tend to make it just a few kilometers down the cable before being lost.

 The team, therefore, used “polarization-preserving quantum frequency conversion” to increase the wavelength of the photons from 780 to 1,517 nanometers, roughly equal to the telecom wavelength 1,550 nanometers – the ideal frequency range for the transmission of light along fiber optics.

This allowed the photons to survive their record-breaking trip down the cable, where they were picked up by a receiver. At this point, a joint measurement was taken of the photons, thereby entangling them. Because each photon was already entangled with the rubidium atom from which it was emitted, this process ultimately caused the two atoms to become entangled with one another.

Once entangled, the two atoms have the potential to act as “quantum memory” nodes within a wider communication network. Importantly, the fact that this was achieved using fiber optic cables raises the possibility of creating such a network using existing telecoms infrastructures.

“The significance of our experiment is that we actually entangle two stationary particles – that is to say, atoms that function as quantum memories,” said lead author Tim van Leent in a statement. “This is much more difficult than entangling photons, but it opens up many more application possibilities.” 

More precisely, co-author Harald Weinfurter explained that “the experiment is an important step on the path to the quantum internet based on existing fiber optic infrastructure.”