‘Spooky action at a distance’ is still spooky no matter how you explain it. Einstein famously used the phrase to describe quantum entanglement, where two entangled particles appear to interact instantaneously even though separated in space. Now we’re talking about using the effect for communications, following the news that European scientists have proven that entanglement persists over a distance of 144 kilometers.
Fortunately for would be communicators, a pair of entangled photons can be created in a process called Spontaneous Parametric Down Conversion. Once entangled, the photons stay entangled until one of them interacts with a third particle. When that happens, the other photon changes its quantum state instantaneously. The beauty of entanglement for communications is that anyone trying to listen in on a message invariably disrupts the entangled system, a result that would be easily detectable.
The security potential is obvious in a world where so much banking information takes digital form, and where the security needs of military communications are greater than ever. But is entanglement a theoretical exercise or can it operate in real-world conditions? To find out, the researchers needed to learn not just how far the effect could travel but also how it might be affected by local conditions. Would it be possible for a ground station, for example, to communicate with an orbiting satellite? Or would the atmosphere destroy the entanglement effect?
To find out, the European team used the European Southern Observatory’s one-meter instrument on Tenerife (Canary Islands), situated 144 kilometers from an observatory on the nearby island of La Palma. The entangled pair was created on La Palma, with one photon sent toward Tenerife while the other remained at La Palma for comparison and study. The entanglement survived, implying that a ground-to-orbit connection is workable.
“We were sending the single-photon beam on a 144 kilometres path through the atmosphere, so this horizontal quantum link can be considered a ‘worst case scenario’ for a space to ground link,” says Josep Perdigues, ESA’s Study Manager. Up next: studying quantum entanglement at much greater distances, something that might be done by putting a quantum optical terminal on a dedicated satellite.
We’ll follow that mission concept as it develops. Meanwhile, theorists still have their work cut out for them. Just why does entanglement survive a journey through a medium in which it might be expected to interact with atmospheric molecules? We have much to learn about such bizarre effects, but the recent demonstration of a workable quantum computer from D-Wave Systems highlights how swiftly ‘spooky’ quantum properties are being harnessed for work in the macroscopic world around us.