10 billion bits of entanglement achieved in silicon

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(PhysOrg.com) -- Scientists from Oxford University have made a significant step towards an ultrafast quantum computer by successfully generating 10 billion bits of quantum entanglement in silicon for the first time– entanglement is the key ingredient that promises to make quantum computers far more powerful than conventional computing devices.

The researchers used high magnetic fields and low temperatures to produce entanglement between the electron and the nucleus of an atom of phosphorous embedded in a highly purifiedsiliconcrystal. The electron and the nucleus behave as a tiny magnet, or 'spin', each of which can represent a bit of quantum information. Suitably controlled, these spins can interact with each other to be coaxed into an entangled state– the most basic state that cannot be mimicked by a conventional computer.

An international team from the UK, Japan, Canada and Germany, report their achievement in this week's Nature.

‘The key to generating entanglement was to first align all the spins by using high magnetic fields and low temperatures,’ said Stephanie Simmons of Oxford University’s Department of Materials, first author of the report.‘Once this has been achieved, the spins can be made to interact with each other using carefully timed microwave and radiofrequency pulses in order to create the entanglement, and then prove that it has been made.’

The work has important implications for integration with existing technology as it uses dopant atoms in silicon, the foundation of the modern computer chip. The procedure was applied in parallel to a vast number of phosphorous atoms.

‘Creating 10 billion entangled pairs in silicon with high fidelity is an important step forward for us,’ said co-author Dr John Morton of Oxford University’s Department of Materials who led the team.‘We now need to deal with the challenge of coupling these pairs together to build a scalable quantum computer in silicon.’

In recent yearsquantum entanglementhas been recognised as a key ingredient in building new technologies that harness quantum properties. Famously described by Einstein as“spooky action at distance”– when two objects are entangled it is impossible to describe one without also describing the other and the measurement of one object will reveal information about the other object even if they are separated by thousands of miles.

Creating true entanglement involves crossing the barrier between the ordinary uncertainty encountered in our everyday lives and the strange uncertainties of the quantum world. For example, flipping a coin there is a 50% chance that it comes up heads and 50% tails, but we would never imagine the coin could land with both heads and tails facing upwards simultaneously: a quantum object such as the electron spin can do just that.

Dr Morton said:‘At high temperatures there is simply a 50/50 mixture of spins pointing in different directions but, under the right conditions, all the spins can be made to point in two opposing directions at the same time. Achieving this was critical to the generation of spinentanglement.’

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Quantum quirk contained: Discovery moves quantum networks closer to reality

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Researchers at the University of Calgary, in Canada, collaborating with the University of Paderborn, in Germany, are working on a way to make quantum networks a reality and have published their findings in the journal<i>Nature</i>. A similar finding by a group at the University of Geneva, in Switzerland is reported in the same issue.

“We have demonstrated, for the first time, that a crystal can storeinformationencoded into entangled quantum states of photons,” says paper co-author Dr. Wolfgang Tittel of the University of Calgary’s Institute for Quantum Information Science.“This discovery constitutes an important milestone on the path toward quantum networks, and will hopefully enable building quantum networks in a few years.”

In current communication networks, information is sent through pulses of light moving through optical fibre. The information can be stored on computer hard disks for future use.

Quantum networks operate differently than the networks we use daily.

“What we have is similar but it does not use pulses of light,” says Tittel, who is a professor in the Department of Physics and Astronomy at the University of Calgary.“In quantum communication, we also have to store and retrieve information. But in our case, the information is encoded into entangled states of photons.”

In this state, photons are“entangled,” and remain so even when they fly apart. In a way, they communicate with each other even when they are very far apart. The difficulty is getting them to stay put without breaking this fragile quantum link.

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Wolfgang Tittel of the University of Calgary is researching ways of integrating quantum memory with current telecommunication technology. Credit: Riley Brandt/University of Calgary

To achieve this task, the researchers used a crystal doped with rare-earth ions and cooled it to -270 Celsius. At these temperatures, material properties change and allowed the researchers to store and retrieve these photons without measurable degradation.

An important feature is that this memory device uses almost entirely standard fabrication technologies.“The resulting robustness, and the possibility to integrate the memory with current technology such as fibre-optic cables is important when moving the currently fundamental research towards applications.”

Quantum networkswill allow sending information without being afraid of somebody listening in.

“The results show that entanglement, a quantum physical property that has puzzled philosophers and physicists since almost hundred years, is not as fragile as is generally believed,” says Tittel.

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