Stopping atoms: researchers minimize laser cooling

It’s cool to be small. Scientists from the National Institute of Standards and Technology (NIST) have miniaturized the optical components needed to cool atoms to a few thousandths of a degree above absolute zero, the first step being to use them on microchips to ‘ a new generation of super accurate atomic clocks, enabling navigation without GPS and simulating quantum systems.

The cooling of atoms is tantamount to slowing it down, making it much easier to study. At room temperature, atoms ferment at almost 343 meters per second through the air. The fast, randomly moving atoms have only volatile interactions with other particles, and their motion can make it difficult to measure transitions between atomic energy levels. When atoms become slow – about 0.1 meters per second – researchers can measure the energy transitions and other quantum properties of the particles accurately enough to be used as reference standards in a multitude of navigation and other devices.

For more than two decades, scientists have cooled atoms by bombarding them with laser bodies, an achievement for which Bill Phillips, a physicist from NIST, shared the Nobel Prize in Physics in 1997. Although laser light will usually move atoms, they move faster, if the frequency and other properties of the light are chosen carefully, the opposite happens. When the atoms are hit, the laser photons reduce the atoms’ momentum until they move slowly enough to be captured by a magnetic field.

But to prepare the laser light in such a way that it has the properties of cooling atoms, an optical composition as large as a dining room table is usually required. This is a problem because it limits the use of these ultra-cold atoms outside the laboratory, where it can become an important element of highly accurate navigation sensors, magnetometers and quantum simulations.

Now NIST researcher William McGehee and his colleagues have designed a compact optical platform, only about 15 centimeters (5.9 inches) long, that cools and traps gas atoms in a 1 centimeter wide area. Although other miniature cooling systems are built, they are the first to rely solely on flat, or flat, optical, which is easy to manufacture.

“This is important because it shows a way to make real devices and not just small versions of lab experiments,” McGehee said. The new optical system, although still about ten times the size of a microchip, is an important step towards the use of ultra-cold atoms in a number of compact, disk-based navigation and quantum devices outside a laboratory environment. Researchers from the Joint Quantum Institute, a collaboration between NIST and the University of Maryland in College Park, along with scientists from the University of Maryland’s Institute for Research in Electronics and Applied Physics, also contributed to the study.

The device, which is online in the New Journal of Physics, consists of three optical elements. First, light is launched from an optical integrated circuit using a device called the extreme mode converter. The converter magnifies the narrow laser beam, initially about 500 nanometers (nm) in diameter (about five thousandths of the thickness of a human hair), to 280 times the width. The enlarged beam then strikes a carefully designed, ultra-thin film known as a ‘meta-surface’ studded with small pillars, approximately 600 nm long and 100 nm wide.

The nanopilars work to further magnify the laser beam by another factor of 100. The dramatic widening is necessary to effectively interact with and cool a large set of atoms. In addition, by achieving performance within a small area of ​​space, the meta-surface reduces the cooling process.

The meta-surface reforms the light in two other important ways, while at the same time changing the intensity and polarization (vibration direction) of the light waves. Usually the intensity follows a bell-shaped curve in which the light is brightest in the center of the beam, with a gradual drop on either side. The NIST researchers designed the nanopilars so that the small structures change intensity and create a beam with a uniform brightness over its entire width. The uniform brightness allows the available light more efficiently. Polarization of light is also critical for laser cooling.

The expanding, reformed beam then strikes a diffraction grating which divides the single beam into three pairs of equal and oppositely directed beams. Combined with an applied magnetic field, the four beams, which push the atoms in opposite directions, serve to capture the cooled atoms.

Each component of the optical system – the converter, the meta-surface and the grid – was developed at NIST, but was in operation in different laboratories on the two NIST campuses in Gaithersburg, Maryland and Boulder, Colorado. McGehee and his team brought the various components together to build the new system.

“This is the funniest part of this story,” he said. “I knew all the NIST scientists who worked independently on these different components, and I realized that the elements could be put together to create a miniature laser cooling system.”

Although the optical system will have to be ten times smaller than laser-cooled atoms on a disk, the experiment is proof of the principle that it can be done, ‘McGehee added.

“Ultimately, the manufacture of the light preparation will be smaller and less complicated to be based on laser cooling technologies outside laboratories,” he said.