Stop atoms: researchers miniaturize laser cooling

It’s cool to be small. Scientists at 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 in their use on microchips to drive a new generation of super-precise atomic clocks, allow navigation without GPS and simulate quantum systems.

Cooling atoms is tantamount to slowing them down, which makes them much easier to study. At room temperature, atoms whistle through the air at a speed almost equal to that of sound – about 343 meters per second. Fast, randomly moving atoms have only fleeting interactions with other particles, and their movement can make it difficult to measure transitions between atomic energy levels. When atoms slow to crawl – around 0.1 meters per second – researchers can measure the energy transitions of particles and other quantum properties with enough precision to use them as reference standards in a myriad of navigation devices and other.

For more than two decades, scientists have cooled atoms by bombarding them with laser light, a feat for which NIST physicist Bill Phillips shared the 1997 Nobel Prize in Physics. Although laser light usually energizes atoms, making them move faster, if the frequency and other properties of light are chosen carefully, the opposite happens. By hitting atoms, laser photons reduce the momentum of atoms until they are moving slowly enough to be trapped by a magnetic field.

But to prepare laser light to have the properties of cooling atoms, you usually need an optical assembly as large as a dining table. This is a problem because it limits the use of these ultra-cold atoms outside of the lab, where they could become a key part of very accurate navigation sensors, magnetometers and quantum simulations.

Now, NIST researcher William McGehee and his colleagues have developed a compact optical platform, about 15 centimeters (5.9 inches) long, that cools and traps gaseous atoms in a region of 1 centimeter of large. Although other miniature cooling systems were built, this is the first that relies solely on flat or planar optics, which are easy to mass produce.

“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, while still about 10 times too big to fit on a microchip, is a key step towards using ultra-cold atoms in a host of compact, chip-based navigation and quantum devices. outside of a laboratory. Researchers from the Joint Quantum Institute, a collaboration between NIST and the University of Maryland at College Park, as well as scientists from the University of Maryland’s Institute for Applied Electronics and Physics Research, also contributed to the study.

The device, described online in the New Physics Journal, consists of three optical elements. First, light is launched from an optical integrated circuit using a device called an extreme mode converter. The converter enlarges the narrow laser beam, initially about 500 nanometers (nm) in diameter (about five thousandths the thickness of a human hair), to 280 times that width. The enlarged beam then hits a carefully crafted ultra-thin film, known as the “metasurface,” studded with tiny pillars, approximately 600nm in length and 100nm in width.

The nanopillars work to further widen the laser beam by a further factor of 100. The dramatic widening is necessary for the beam to effectively interact with and cool a large collection of atoms. Additionally, by accomplishing this feat in a small region of space, the metasurface miniaturizes the cooling process.

The metasurface reshapes light in two other important ways, simultaneously changing the intensity and polarization (direction of vibration) of 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 decrease on either side. NIST researchers designed the nanopillars so that the tiny structures change intensity, creating a beam that has uniform brightness across its width. Uniform brightness allows more efficient use of available light. The polarization of the light is also critical for cooling the laser.

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

Each component of the optical system – the converter, the metasurface, and the array – had been developed at NIST but operated in separate labs on NIST’s two campuses, in Gaithersburg, Maryland and Boulder, Colorado. McGehee and his team brought the disparate components together to build the new system.

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

Although the optical system will have to be 10 times smaller to laser cool the atoms on a chip, the experiment “is proof in principle that it can be done,” McGehee added.

“Ultimately, making light preparation smaller and less complicated will allow technologies based on laser cooling to exist outside of labs,” he said.