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Posted November 25, 2015

Quantum entanglement is the observed phenomenon of two or more particles that are connected, even over thousands of miles. By harnessing this behaviour, which even Albert Einstein called “spooky action”, scientists hope to develop quantum networks that would be capable of transmitting highly secure information over long distances.

This nonlinear optical wave guide converts the wavelength of a single-photon signal to a common telecom wavelength. Image courtesy of Stanford University.


While being bound by atoms effectively blocks direct communication between electrons, the same cannot be said of photons. Scientists can establish a necessary condition of entanglement, called quantum correlation, to match photons to electrons, making the former act as messengers of the latter’s spin.

This was proven in a former study by Stanford physicist Leo Yu, who’d successfully entangled photons with electrons through fibre optic cables over a distance of several feet. Now, teamed up with a group of other researchers at the university, he managed to accomplish the same over a record distance of 1.2 miles.

The main obstacle to overcome was the tendency of photons to change orientation while traveling through the cables – if a photon changes its orientation en route, the connection to the correlated electron is lost.

To preserve this information, Yu created a time-stamp to correlate arrival time of the photon with the electron spin, which provided a sort of reference key for each photon to confirm its correlation to the source electron.

Entangling two electrons that had never met over great distances necessitated the sending of two photons through fibre optic cables to meet in the middle at a “beam splitter”. Given that photons from different sources cannot interact due to having different characteristics, Yu made them pass through a “quantum down-converter” before they reached the fibre optic cable, which matched their wavelengths.

The down-converter also shifted both photons to a wavelength that can travel farther within the fibre optic cables designed for telecommunications.

Quantum supercomputers promise to be exponentially faster and more powerful than traditional computers, and could communicate with immunity to hacking or spying. “This work can pave the way for future quantum networks that can send highly secure data around the world,” said Yu.


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