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Optical Fibers

Graphic depiction of higher mode of light traveling in a tapered optical fiber. Credit: JQI

How do fiber optics work? Light can be confined inside a reflective medium—a stream of water, a thread of glass fiber. The light moves, trapped in these materials via total internal reflection--light “totally” bounces at the surfaces, back and forth. Certain wavelengths of light can travel over vast distances without much loss or signal degradation. Thus information encoded using different attributes of the light (phase, frequency) can be transmitted efficiently. 

How is this used in Quantum Information Research

Fibers are a standard lab tool used mainly to transport and combine laser light over short and long distances. They provide stability through isolation, replacing mechanical "free-space" optical elements like mirrors. They also have other research-specific uses. Read on to learn about how fibers can be used to trap atoms.

Recall that light travels in fiber by bouncing back-and-forth off the surface. If you zoom-in on what's happening with the electromagnetic light waves at the surface, you would encounter is what’s called an evanescent field. Webster says “evanescent” means “tending to dissipate, like vapor.” In physics, an evanescent wave is a vanishing vibration, occurring at an interface. They occur because nature doesn’t like discontinuity. The evanescent field isn’t the same as leaky, inefficient fibers; it is always present as light moves through the fiber.

“Dissipating, like vapor” doesn’t sound like something useful, but in fact, under certain conditions, evanescent waves can be used to isolate and probe atoms. Atoms trapped in the evanescent wave can interact with light traveling through the fiber. The information from that interaction is not lost; the light then re-enters, or “couples,” back into the fiber and can be captured on a camera.

The fibers in communications are relatively large compared to the wavelength of light, about 10 to 100 times greater depending on the length and use of the fiber. Harnessing evanescent fields requires shrinking the fiber core. Here the team heats up a fiber while simultaneously stretching it—a technique reminiscent of glass blowing—down to a diameter of 350 nm. This is more than two times smaller than the light’s wavelength (780 nm), which means that somewhere along the line, the light can’t completely fit inside the fiber. Instead, its electric field is mostly outside the core—where it can interact with atoms, for instance.