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Physicists set quantum network record over 1.2 mile-long optic fibre

30 November 2015

An international team of researchers has advanced a long-standing problem in quantum physics – how to send 'entangled' particles over long distances.

Leo Yu, left, with senior research scientist Carsten Langrock (photo: L A Cicero)

Scientists and engineers are interested in the practical application of this technology to make quantum networks that can send highly secure information over long distances – a capability that also makes the technology appealing to governments, banks and military organisations.

Quantum entanglement is the observed phenomenon of two or more particles that are connected, even over thousands of miles - a phenomenon once described by Albert Einstein as "spooky action".

In the case of entangled electrons, electrons spin in one of two characteristic directions, and if they are entangled, those two electrons' spins are linked. Electrons are trapped inside atoms, so entangled electrons can't 'talk' directly at long distance. However, photons can move, and scientists can establish a necessary condition of entanglement, called quantum correlation, to correlate photons to electrons, so that the photons can act as the messengers of an electron's spin.

In his previous work, Stanford physicist Leo Yu entangled photons with electrons through fibre optic cables over a distance of several feet. Now, he and a team of scientists, including Professor Emeritus Yoshihisa Yamamoto  and University of Glasgow post-doctoral research fellow Dr Chandra Mouli Natarajan, have correlated photons with electron spin over a record distance of 1.2 miles.

"Electron spin is the basic unit of a quantum computer," Yu says. "This work can pave the way for future quantum networks that can send highly secure data around the world."

To do this, Yu and his team had to make sure that the correlation could be preserved over long distances – a key challenge given that photons have a tendency to change orientation while travelling in optical fibres.

This nonlinear optical wave guide converts the wavelength of a single-photon signal to a common telecom wavelength (photo: L A Cicero)

Photons can have a vertical or horizontal orientation (known as polarization), which can be referenced as a '0' or a '1', as in digital computer programming. But if they change en route, the connection to the correlated electron is lost.  

This information can be preserved in another way. Yu created a time-stamp to correlate arrival time of the photon with the electron spin, which provided a reference key for each photon to confirm its correlation to the source electron.

To entangle two remote electrons over great distances, two photons, each correlated with a unique source electron, has to be sent through fibre optic cables to meet in the middle at a 'beam splitter' and interact. Photons do not normally interact, so the researchers had to mediate this interaction called the 'two-photon interference'.

To ensure two-photon interference, they had another issue to overcome. Photons from two different sources have different characteristics, like colour and wavelength, and if they have different wavelengths, they cannot interfere. Before travelling along the fibre optic cable, the photons passed through a 'quantum down-converter', which matched their wavelengths. The down-converter also shifted both photons to a wavelength that can travel farther within fibre optic cables designed for telecommunications.

Quantum supercomputers promise to be exponentially faster and more powerful than traditional computers and can communicate with immunity to hacking or spying. With this work, the team, co-ordinated by University of Glasgow post-doctoral research fellow Dr Chandra Mouli Natarajan, believes it has brought quantum networks one step closer to reality.

The work, involving researchers from the Universities of Glasgow, Stanford, Tokyo and Würzburg, is described in the online edition of Nature Communications.

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