Transporting Entangled Quantum State Between Atom and Photon via Optic Fiber

In collaboration with researchers at Saarland University, physicists from LMU have now set a new record. They have been successful in transporting an entangled state between a photon and an atom through an optic fiber over a distance of up to 20 km.

Picture of the single atom trap. In the ultra-high vacuum glass cell, a single Rubidium atom is captured, which later will be entangled with a photon. Image Credit: C. Olesinski/LMU.

“Entanglement” represents a highly specific type of quantum state, unattributed to just a single particle, but shared between two distinct particles. It irreversibly links their consequent fates together—regardless of how distant they are from each other—which notably led Albert Einstein to term the phenomenon as “spooky action at a distance.”

Entanglement has turned out to be a foundation for new technologies based on quantum-level effects, and transportation over long distances is the main objective in quantum communication.

Led by physicist Harald Weinfurter, researchers from LMU, in collaboration with researchers at the Saarland University in Saarbrücken, have now demonstrated that it is possible to transmit the entangled state of a photon and an atom via an optic fiber (such as those used in telecommunications networks) over a distance of up to 20 km. The earlier record was 700 m.

The experiment represents a milestone, insofar as the distance covered confirms that quantum information can be distributed on a large scale with little loss. Our work therefore constitutes a crucial step toward the future realization of quantum networks.

Harald Weinfurter, Physicist, LMU

Quantum networks typically include quantum memories (constituting one or more atoms, for instance) acting as nodes and communication channels where photons (light quanta) can travel to link the nodes together.

As part of the experiment, a rubidium atom was entangled by the team with a photon, thus being able to detect the entangled state—which consequently shares the quantum properties of both the particles—once it passes via a 20-km optic fiber coil.

The greatest challenge faced by the researchers begins with the properties of the rubidium atom. After targeted excitation, these atoms discharge photons that have a wavelength of 780 nm, in the near-infrared region of the spectrum.

In an optic fiber made of glass, light at this wavelength is rapidly absorbed.

Harald Weinfurter, Physicist, LMU

Thus, traditional telecommunications networks use wavelengths of around 1550 nm, which significantly minimizes losses in transit.

Evidently, this wavelength would also enhance the researchers’ chances of success. Therefore, Matthias Bock, a member of the team from Saarbrücken, developed a so-called quantum frequency converter with an exclusive design that increases the wavelength of the emitted photons from 780 to 1520 nm.

This task led to several highly demanding technical difficulties. This is because it was essential to make sure that conversion from only one photon to only one other photon occurs and that no other properties of the entangled state, specifically the polarization of the photon, were modified at the time of the conversion process. Otherwise, there would be a loss of entangled state.

Thanks to the use of this highly efficient converter, we were able to maintain the entangled state over a much longer range at telecommunications wavelengths, and therefore to transport the quantum information that it carries over long distances.

Harald Weinfurter, Physicist, LMU

In the future, the experimenters intend to frequency-convert the light discharged by a second atom, whereby they can produce entanglement between the two atoms over long telecommunications fibers. The properties of glass-fiber cables differ based on factors like the strain and temperature to which they are subjected.

Therefore, the researchers plan to first perform this experiment under controlled conditions in the lab. Upon gaining success, field experiments will be performed, which will add new nodes to a growing network. Eventually, it is possible to successfully complete even long journeys by covering one step at a time.


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