Sensing Atoms Caught in Ripples of Light

Cartoon graphic of a nanoscale optical fiber immersed in a chilled gas of atoms. Atoms from the gas are caught by the wiggles of light that surround the narrowest part of the fiber. Light waves propagating down the fiber both capture and probe the atoms, allowing scientists to study interesting light-matter physics. (Source: E. Edwards, JQI)

Optical fibers are ubi­quitous, carrying light wherever it is needed. These glass tunnels are the high-speed railway of infor­mation transit. Fibers are also thin and flexible, so they can be immersed in many different environ­ments, including the human body, where they are employed for illu­mination and imaging. Physicists use fibers, too, parti­cularly those who study atomic physics and quantum infor­mation science. Aside from shuttling laser light around, fibers can be used to create light traps for super-chilled atoms. Captured atoms can interact more strongly with light, much more so than if they were moving freely. This rather arti­ficial environ­ment can be used to explore funda­mental physics questions, such as how a single particle of light interacts with a single atom. But it may also assist with deve­loping future hybrid atom-optical tech­nologies.

Now, researchers from the Joint Quantum Insti­tute and the Army Research Labora­tory have developed a fast-acting, non-inva­sive way to use fiber light to reveal information about fiber traps. This technique is reminiscent of bio­medical and chemical sensors that use fibers to detect proper­ties of nearby molecules. Fiber sensors are an attrac­tive measure­ment tool because they can often extract infor­mation without totally disrupting interes­ting pheno­mena that may be going on.

Typical optical fibers, like the ones used in communi­cations and medicine, have only a tiny amount of light near the outside surface, and that is not enough to capture atoms from a sur­rounding gas. Physicists can push more light to the outside by reshaping the fiber to look like a tiny hourglass instead of a tunnel. The waist of the hourglass is hundreds of nano­meters, a few times the width of a human hair and too small to contain light waves that are propa­gating along the inside of the fiber. But instead of just stopping at the con­striction, the light squeezes to the outside surface. When physicists inject light into both ends of such a fiber, the light waves combine together to form a sta­tionary ripple around the con­striction. Atoms will be attracted to dips in the wave and line up like a row of eggs in a carton.

This trapping is an example of how light affects atoms, drawing them in. But the light-atom relation­ship is reciprocal: The presence of atoms can alter the light, too. Light waves, sent into one end of a nanoscale fiber, will pick-up infor­mation about the atoms in the vici­nity of the fiber, and then convey it to a detector at the opposite end of the fiber. Each trapped atom acts like a marble in a glass bowl. When pushed, a marble will roll up the side of the bowl, back down, and then up the other side. The speed of this cycle is related to the bowl’s curvature: Steeper walls cause faster cycles. Now imagine shining a flashlight through one side of the bowl. As it goes back and forth the marble will keep passing through the flash­light beam. The beam signal will blink on and off at the rate at which the marble was moving in the bowl. In other words, the information about the marble motion, and therefore the bowl’s shape, is encoded onto the flash­light beam.

In this research, the team uses laser light as the probe, analogous to the flashlight. A mere 70 nanowatts in power is injected into the fiber, gently kicking the atoms into motion. Similar to marble wobbles, the atoms rock back and forth in their bowl traps. Instead of causing the probe light to blink on and off, the atom motion affects the direc­tion that the light waves oscil­late. The speed of the atom rocking, which is directly related to the atom trap shape, will be imprinted on the light as faster or slower changes. When the light waves complete their journey and exit the fiber, the team catches them with a detector to conti­nuously monitor the atom-light oscil­lations. The process is fast, taking only a fraction of a milli­second, and it can be seamlessly integrated into an experi­mental sequence.

When it comes to measuring these atom trap properties, physicists want to avoid distur­bances. This can be difficult to do because one of the most effective ways to probe atoms involves blasting them with light, which can heat and even release them from their traps. This conven­tional method is accep­table because scientists can just recool and recapture the atoms. In contrast, the JQI-ARL technique uses very little light and is done in-situ, meaning it collects infor­mation while mini­mizing disruptions. This appealing alter­native promises to streamline atom-fiber experi­ments. (Source: JQI)

Reference: P. Solano et al.: Dynamics of trapped atoms around an optical nanofiber probed through polarimetry, Opt. Lett. 42, 2283 (2017); DOI: 10.1364/OL.42.002283

Link: Joint Quantum Institute JQI, Dept. of Physics, University of Maryland, College Park, USA

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