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When Rupert Ursin stood in the darkness at the highest point of La Palma in the Canary Islands he found it scary. "Really scary," he says. It was less the blackness stretching out towards the Atlantic Ocean some 15 kilometres away. It was more the sheer technical challenge ahead- and perhaps just a little because of the ghosts he was attempting to lay to rest.

Ursin and his colleagues from the Institute for Quantum Optics and Quantum Information in Vienna, Austria, were there that night to see if they could beam single photons of light to the 1-metre aperture of a telescope on the island of Tenerife, 144 kilometres away. Even on a fine day, when Teide, Tenerife's volcanic peak, is clearly visible from La Palma, that would be a feat of mind-boggling precision. Attempting it in the dark seemed ludicrous. "At night you don't know where the other island is," says Ursin. "You are lost; you have no clue what to do."

In daylight, though, zillions of photons zinging around would have made their experiment impossible. And so, on moonless nights, the researchers would switch off the lights in their lab and slip outside to a night sky lit only by the Milky Way.

For what? To attempt to settle one of the longest-running debates in modern physics. To dispose of yet another ambiguity in our basic understanding of how nature ticks. To answer one of the most fundamental questions of all: is quantum reality real?

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Category: Science