A secure quantum internet is one step closer thanks to a quantum memory made from a crystal, which could form a crucial part of a device able to transmit entangled photons over a distance of 5 kilometres. Crucially, it is entirely compatible with existing communication networks, making it suitable for real-world use.
There has long been a vision of a quantum version of the internet, which would allow quantum computers to communicate across long distances by exchanging particles of light called photons that have been linked together with quantum entanglement, allowing them to transmit quantum states.
The problem is that photons get lost when they are transmitted through long lengths of fibre-optic cable. For normal photons, this isn’t an issue, because networking equipment can simply measure and retransmit them after a certain distance, which is how normal fibre data connections work. But for entangled photons, any attempt to measure or amplify them changes their state.
The solution to this is a procedure called quantum teleportation. This involves simultaneously measuring the state of one photon from each of two pairs of entangled photons, which effectively links the most distant two photons in the chain.
“The photons are used not to send the information, but to share the entanglement. Then I can use that entanglement. I can teleport the quantum information I want from A to B,” says Myungshik Kim at Imperial College London.
But that introduces another problem – all of your entangled pairs have to be ready at the same time to form a chain, which becomes more difficult over longer distances. To solve this, you need a quantum memory.
“The idea is that you try one link, and when you have a success, then you stall this entanglement and this link and you wait for the other link to be also ready. And when the other links are ready, then you can combine them together. This will extend the entanglement towards larger and larger distances,” says Hugues de Riedmatten at the Institute of Photonic Sciences in Castelldefels, Spain.
De Reidmatten and his team used yttrium orthosilicate crystals to store pairs of entangled photons for 25 microseconds in two separate quantum memories. They performed the experiment between two labs, linked by 50 metres of fibre-optic cable, but theoretically this amount of storage time would allow devices up to 5 kilometres apart to communicate.
Crucially, the researchers were able to store and retrieve photons in the order they were sent, and transmit them using frequencies and fibre-optic cables already used in data networks, showing that the approach should work outside the lab. They now hope to increase the distance between the two memory devices by increasing the maximum storage time and make a fully functioning quantum repeater.
Journal reference: Nature, DOI: 10.1038/s41586-021-03481-8
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