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Crucial connection for 'quantum internet' made for the first time

The ability to share quantum information is crucial for developing quantum networks for distributed computing and secure communication. Quantum computing will be useful for solving some important types of problems, such as optimising financial risk, decrypting data, designing molecules, and studying the properties of materials.

However, this development is being held up because quantum information can be lost when transmitted over long distances. One way to overcome this barrier is to divide the network into smaller segments and link them all up with a shared quantum state

To do this requires a means to store the quantum information and retrieve it again: that is, a quantum memory device. This must 'talk' to another device that allows the creation of quantum information in the first place.

For the first time, researchers have created such a system that interfaces these two key components, and uses regular optical fibres to transmit the quantum data.

The feat was achieved by researchers at Imperial College London, the University of Southampton, and the Universities of Stuttgart and Wurzburg in Germany, with the results published in Science Advances.

Co-first author Dr Sarah Thomas, from the Department of Physics at Imperial College London, said: "Interfacing two key devices together is a crucial step forward in allowing quantum networking, and we are really excited to be the first team to have been able to demonstrate this."

Co-first author Lukas Wagner, from the University of Stuttgart, added: "Allowing long-distance locations, and even to quantum computers, to connect is a critical task for future quantum networks."

Long-distance communication

In regular telecommunications -- like the internet or phone lines -- information can be lost over large distances. To combat this, these systems use 'repeaters' at regular points, which read and re-amplify the signal, ensuring it gets to its destination intact.

Classical repeaters, however, cannot be used with quantum information, as any attempt to read and copy the information would destroy it. This is an advantage in one way, as quantum connections cannot be 'tapped' without destroying the information and alerting the users. But it is a challenge to be tackled for long-distance quantum networking.

One way to overcome this problem is to share quantum information in the form of entangled particles of light, or photons. Entangled photons share properties in such a way that you cannot understand one without the other. To share entanglement over long distances across a quantum network you need two devices: one to create the entangled photons, and one to store them and allow them to be retrieved later.

There are several devices used to create quantum information in the form of entangled photons and to store it, but both generating these photons on demand and having a compatible quantum memory in which to store them eluded researchers for a long time.

Photons have certain wavelengths (which, in visible light, creates different colours), but devices for creating and storing them are often tuned to work with different wavelengths, preventing them from interfacing.

To make the devices interface, the team created a system where both devices used the same wavelength. A 'quantum dot' produced (non-entangled) photons, which were then passed to a quantum memory system that stored the photons within a cloud of rubidium atoms. A laser turned the memory 'on' and 'off', allowing the photons to be stored and released on demand.

Not only did the wavelength of these two devices match, but it is at the same wavelength as telecommunications networks used today -- allowing it to be transmitted with regular fibre-optic cables familiar in everyday internet connections.



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