A team led by Innsbruck physicist Ben Lanyon has sent a light particle entangled with matter over 50 km of optical fibre. This first-time achievement is hailed as paving the way for the practical use of quantum networks, and sets a milestone for a future quantum Internet.
The Innsbruck team note that because quantum information cannot be copied, it is not possible to send this type of information over a classical network. Quantum information must be transmitted by quantum particles, and special interfaces are required for this.
Lanyon is researching these important intersections of a future quantum Internet. Now, his team at the Department of Experimental Physics at the University of Innsbruck, and at the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences, has achieved a record for the transfer of quantum entanglement between matter and light. For the first time, a distance of 50 km was covered using fibre optic cables. “This is two orders of magnitude further than was previously possible, and is a practical distance to start building inter-city quantum networks,” said Lanyon.
Lanyon’s team started the experiment with a calcium atom trapped in an ion trap. Using laser beams, the researchers write a quantum state onto the ion and simultaneously excite it to emit a photon in which quantum information is stored. As a result, the quantum states of the atom and the light particle are entangled.
But the challenge was to transmit the photon over fibre optic cables. “The photon emitted by the calcium ion has a wavelength of 854 nm and is quickly absorbed by the optical fibre,” observed Lanyon. His team therefore initially sends the light particle through a nonlinear crystal illuminated by a strong laser. With this arrangement, the photon wavelength is converted to the optimal value for long-distance travel: the current telecommunications standard wavelength of 1,550 nm. The researchers then send this photon through a 50 km-long optical fibre line. Their measurements show that atom and light particle are still entangled even after the wavelength conversion and this distance.
As a next step, Lanyon and his team show that their methods would enable entanglement to be generated between ions 100 km and more apart. Two nodes each send an entangled photon over a distance of 50 km to an intersection where the light particles are measured in such a way that they lose their entanglement with the ions, which in turn would entangle them. With 100 km node spacing now a possibility, it’s conceivable that the world’s first intercity light-matter quantum network could be built in the next few years. The researchers conclude that only a handful of trapped ion-systems would be required on the way to establish a quantum Internet between Innsbruck and Vienna, for example.
Lanyon’s team is part of the Quantum Internet Alliance, an international project within the Quantum Flagship framework of the European Union. The current results have been published in the Nature journal Quantum Information.
Picture credit: IQOQI Innsbruck/Harald Ritsch
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