Making Quantum Practical

Kate Greene writes in the latest issue of MIT Technology Review about researchers who 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.

The researchers pulsed polarized laser light through 300 meters of coiled fiber. It is this property of polarization (the orientation of the photons) that allows it to become entangled when the pairs of photons are created: if the polarization of one photon is measured, the polarization of the other photon is instantly known. Within the fiber, about one pair of polarization-entangled photons is created every microsecond, Kumar says, and the rate can be increased 100-fold by pulsing the light faster, he adds.

Next, the entangled photons are split apart and each is directed into 50 kilometers of fiber (for a total of 100 kilometers), where they join a classical signal. At the opposite ends of the fibers, the photons are separated from the communication signals, and shoot towards two different photon detectors, built to see one photon at a time. Kumar says he knows he's successfully sent entangled photons when both detectors see certain types of polarized photons at the same time.

There are still challenges to using traditional fiber-optic cable and sending entangled photons 100 kilometers. Even the best quality commercial fiber has very small geometric inconsistencies, Kumar says, which can alter the polarization of the photon pairs slightly, decreasing the quality of entanglement -- and rendering the quantum information useless.

These slight changes in polarization can usually be adjusted for by sending the photons through special polarization devices right before they hit the detector, but it is difficult to know exactly how to adjust these devices to best compensate for the change in polarization. Interestingly, Kumar adds, the classical signal traveling with the quantum signal, as in the experiment, can help. It can track imperfections in the fiber encountered by the entangled photon, and relay this information so the polarization control device can be set to compensate appropriately.

Right now, Kumar's team is working on testing the distance limits of entangled photon transport and determining how many more classical signals they can add to the line and still retrieve the quantum information stored in the entangled photons. Because in real-world fiber optics, multiple signals pass through at once, it would be useful to know how many classical signals can share the fiber with a quantum signal.

According to other scientists working in the field of quantum information, the fact that Kumar's team has combined fiber-generated entangled photons with classical information, and sent the total signal over a record distance in a traditional fiber line is an exciting advance. "Pieces have been shown, but this puts it all together," says Williams, who calls it "a remarkable demonstration."

Jeffrey Shapiro, professor of electrical engineering at MIT, says it is "great work...Prem [Kumar] works both on classical and quantum communication, and is one of the people who's well suited to address both sides."

Ultimately, as quantum information matures, it will become more integrated into traditional fiber technology, says Kumar. "My goal is to make quantum optics applicable," he notes. "Fiber-based quantum optics can piggyback on billions of dollars in optical communications technology. We want to ride that wave."

The researchers pulsed polarized laser light through 300 meters of coiled fiber. It is this property of polarization (the orientation of the photons) that allows it to become entangled when the pairs of photons are created: if the polarization of one photon is measured, the polarization of the other photon is instantly known. Within the fiber, about one pair of polarization-entangled photons is created every microsecond, Kumar says, and the rate can be increased 100-fold by pulsing the light faster, he adds.

Next, the entangled photons are split apart and each is directed into 50 kilometers of fiber (for a total of 100 kilometers), where they join a classical signal. At the opposite ends of the fibers, the photons are separated from the communication signals, and shoot towards two different photon detectors, built to see one photon at a time. Kumar says he knows he's successfully sent entangled photons when both detectors see certain types of polarized photons at the same time.

There are still challenges to using traditional fiber-optic cable and sending entangled photons 100 kilometers. Even the best quality commercial fiber has very small geometric inconsistencies, Kumar says, which can alter the polarization of the photon pairs slightly, decreasing the quality of entanglement -- and rendering the quantum information useless.

These slight changes in polarization can usually be adjusted for by sending the photons through special polarization devices right before they hit the detector, but it is difficult to know exactly how to adjust these devices to best compensate for the change in polarization. Interestingly, Kumar adds, the classical signal traveling with the quantum signal, as in the experiment, can help. It can track imperfections in the fiber encountered by the entangled photon, and relay this information so the polarization control device can be set to compensate appropriately.

Right now, Kumar's team is working on testing the distance limits of entangled photon transport and determining how many more classical signals they can add to the line and still retrieve the quantum information stored in the entangled photons. Because in real-world fiber optics, multiple signals pass through at once, it would be useful to know how many classical signals can share the fiber with a quantum signal.

According to other scientists working in the field of quantum information, the fact that Kumar's team has combined fiber-generated entangled photons with classical information, and sent the total signal over a record distance in a traditional fiber line is an exciting advance. "Pieces have been shown, but this puts it all together," says Williams, who calls it "a remarkable demonstration."

Jeffrey Shapiro, professor of electrical engineering at MIT, says it is "great work...Prem [Kumar] works both on classical and quantum communication, and is one of the people who's well suited to address both sides."

Ultimately, as quantum information matures, it will become more integrated into traditional fiber technology, says Kumar. "My goal is to make quantum optics applicable," he notes. "Fiber-based quantum optics can piggyback on billions of dollars in optical communications technology. We want to ride that wave."