A protocol for entangling microwave and optical photons has been demonstrated by researchers in Austria. This has the potential to help to overcome one of the central issues in the formation of a quantum internet by allowing microwave frequency circuits to exchange quantum information through optical fibres.
The central vision underpinning a quantum internet – first articulated back in 2008 by Jeff Kimble of Caltech in the US – is that networked quantum processors could exchange quantum information, much as classical computers exchange classical information via the Internet. Transferring quantum information is far more difficult, however, because background noise can destroy quantum superpositions in a process called decoherence.
Many of the most powerful quantum computers in existence, such as IBM’s Osprey, use superconducting qubits. These work at microwave frequencies, which makes them extremely vulnerable to disruption by background thermal radiation – and explains why they need to be kept at cryogenic temperatures. It also makes transferring information between superconducting qubits extremely difficult. “[One way] is to build ultracold links,” explains Johannes Fink of the Institute for Science and Technology Austria in Klosterneuburg. “The record was just published in Nature [by Andreas Wallraff’s group at ETH Zurich in Switzerland and colleagues]: 30 m at 10–50 mK – that has some challenges for scaling up.” In contrast, he says, “fibre optics works really well for communication – we use it all the time when we surf the Internet”
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