Editorial Feature

Entanglement is Sent Over 50 km of Optical Fiber


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Researchers in Austria have reported the successful transmission of a light particle entangled with matter across 50 km of optical fiber for the first time ever. The establishment of a practical method to send entanglements has strong implications for the future of the quantum internet. Details on the methodology, published this year in the journal Nature, will undoubtedly be fundamental to instituting a new paradigm for information processing.

Entanglement across Larger Distances Necessary for Quantum Networks

High hopes are held for what a future with the quantum internet would bring. Experts predict that it would foster a safer environment for information transfer, assist in scientific research through carrying out computations that are currently outside of the limitations of conventional computers, and essentially be relied on to solve the biggest issues that face our digital age.

A team of researchers at the Department of Experimental Physics at the University of Innsbruck along with those at the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences set about investigating the quantum entanglements that would usher in the age of the quantum internet.

Already, previous research has demonstrated that entanglement could be achieved between two atoms in traps a few tens of meters apart, between two ions in traps several meters apart, and most recently the entanglement of two nitrogen-vacancy centers 1.3 km apart had been achieved.

This earlier researcher generated their results by initiating photon-matter entanglement, and detecting one or two photons heralds remote matter-matter entanglement. The researchers of the current project planned to scale up this distance to allow for large-scale quantum networks.

Overcoming Obstacles to Send Entanglements Further

The Austrian based team, led by START Prize winner, experimental physicist Ben Lanyon, set about creating a method of sent light particles entangled with matter over record-breaking distances.

Their method began with a calcium atom trapped in an ion trap. They used a 3D radio-frequency linear Paul trap featuring an ion to blade distance of 0.8 mm and a DC endcap to ion separation of 2.5 mm. They selected titanium, coated with gold to make the trap electrodes, which were mounted on Sapphire holders.

Then, the team wrote a quantum state onto the ion with laser beams, while at the same time exciting the ions, resulting in an emission of a photon in which quantum information is stored.

The consequence of this methodology induced an entanglement between the quantum states of the atom and the light particle. However, the difficulty arose in being able to transmit the photo via the fiber optic cables. The challenge was that the photon emitted by the calcium ion had a wavelength of 854 nanometers, meaning it is readily absorbed into the optical fiber, making long-distance travel problematic.

To overcome this, the team first sent the photon through a nonlinear crystal illuminated by a strong laser, having the impact of converting the photon wavelength into the optimal value for long-distance travel.

Following this, the researchers sent the optimized photon across 50km of optical fiber, measuring it to demonstrate the continued entanglement of the atom and light particle for the entire length of the journey.

A World-first Achievement

The method was a success, and the entangled photon achieved traveling the distance of the 50 km fiber, resulting in a record-breaking distance covered by a quantum entanglement between matter and light. The team had managed to send the entanglement across a distance two orders of magnitude further than was possible previously, paving the way for the construction of quantum networks.

Results may Usher in the Future of the Quantum Internet

The study not only proved that sending entanglements over long distances (50 km) is achievable, but their methodology opened the door to further advancements, with the potential to scale up this effect even further being a likely outcome. The researchers believe that their methodology could be used to transport an entanglement over double the distance. The possibly of 100-kilometer node spacing is vital to the development of light-matter quantum networks. What the Austrian team has accomplished here will likely influence future advancements in the field of the quantum internet, which is predicted to rapidly evolve over the coming years.

Sources and Further Reading

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Sarah Moore

Written by

Sarah Moore

After studying Psychology and then Neuroscience, Sarah quickly found her enjoyment for researching and writing research papers; turning to a passion to connect ideas with people through writing.


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