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.”