Making Quantum Practical

Researchers have succeeded in combining quantum signals with classical optical signals in a conventional fiber-optic line.

Coursing through the fiber-optic veins of the Internet are photons of light that carry the fundamental bits of information. Depending on their intensity, these photons represent bits as 1s and 0s. This on-and-off representation of information is part of what physicists call "classical" phenomena.

But photons of light have "quantum" properties as well, which, when exploited, provide more than simply a 1 or 0; these properties allow photons to represent 1s and 0s simultaneously. When information is approached from a quantum perspective, say scientists, encryption can be perfectly secure and enormous amounts of information can be processed at once.

This field of quantum information –- the transmission and processing of data governed by quantum mechanics –- is rapidly moving beyond the lab and into the real world. Increasingly, researchers are conducting experiments within the same commercial fiber that transmits information in the classical way. For the most part, though, the two types of information have not intermingled: quantum information has been sent only over dedicated fiber.

Now researchers at Northwestern University have shown that quantum information, in the form of "entangled photons," can travel over the same fiber as classical signals. Additionally, the researchers have sent the combination signal through 100 kilometers of fiber -- a record distance for entangled photons even without the classical signal.

This marriage of quantum and classical optics shows that traditional optical tools can be used to send quantum encryption keys, based on entangled photons (some other schemes rely on single photons). In the future, this new technique might also enable long-distance networking between quantum computers, says Carl Williams, coordinator of the Quantum Information Program at the National Institute of Standards and Technology in Gaithersburg, MD.

At the heart of the Northwestern experiment are the entangled photons: pairs of photons with interconnected properties. That is, looking at one photon in an entangled pair will reveal what the result of looking at the other photon would be -- no matter how far apart the photons are. Entangled photons can be used in encryption by encoding information about a key in the photons. Then if an eavesdropper intercepts one photon of the entangled pair, the entire transmission is altered, alerting the code makers.

Furthermore, entangled photons used for quantum computing could be split up and shared across a network of many quantum computers. Such photon pairs are "important whether the application is cryptography or anything else," says Prem Kumar, professor of electrical and computer engineering and physics at Northwestern and lead scientist on the project.

The first step in the experiment, then, was for the researchers to create entangled photons. Traditionally, shining laser light into a type of crystal has produced entangled photons. But it's been difficult to use entangled photons made from crystals, because in transferring them into a fiber, you "lose the quality of the entanglement," says Williams.

Instead, Kumar's team created their photon pairs by exploiting a similar, recently developed process that can occur within long lengths of standard fiber. The photons start in fiber and remain in it for the duration of the experiment, retaining their entanglement properties.