Miniaturization of Optical Components Enables Atom Cooling

National Institute of Standards and Technology (NIST) researchers miniaturized the optical components necessary for atom cooling, achieving a step toward employing the technology on microchips. The research holds implications for atomic clocks, navigation without GPS, and the simulation of quantum systems.

To cool atoms is to slow them down, which enables them to be studied more closely. At room temperature, atoms travel close to the speed of sound, which makes the study of atomic interactions and transitions between atomic energy levels difficult. When cooled to nearly absolute zero, the speed can drop down to about 0.1 m/s, enabling energy transitions and other quantum properties to be measured accurately enough for use as reference standards.

To slow atoms down, researchers use lasers, the light from which can be tuned in such a way that, upon contact, the photons reduce the atoms’ momentum until they are moving slowly enough to be trapped by a magnetic field. This procedure, however, typically requires an optical assembly that researchers say is about the size of a dining room table. The size prohibits the use of ultracold atoms outside of a laboratory setting.

The NIST researchers report that they have crafted a compact platform, approximately 15 cm in length, that cools and traps gaseous atoms in a 1-cm-wide region. While other miniature cooling systems have been developed, the researchers claim that this is the first to rely solely on flat, or planar optics, which are easier to produce in mass.

“This is important as it demonstrates a pathway for making real devices and not just small versions of laboratory experiments,” said William McGehee, a physicist in NIST’s atomic devices and instrumentation group.

Though the device is about 10 times too large to be able to fit on a microchip at present, it is a key step toward employing ultracold atoms in compact, chip-based navigation and quantum devices outside a laboratory setting. The device emits light from an optical integrated circuit with an extreme mode converter that enlarges the laser beam, approximately 500 nm in diameter, by about 280 times. The widened beam then passes through a metasurface about 600 nm in length and 100 nm wide.

The nanopillars of the metasurface are arranged to further widen the beam by an additional factor of 100. That widening allows the beam to interact with and cool a large collection of atoms. The light is reshaped in two other ways, by altering the intensity and polarization of the lightwaves. Typically, laser light follows a bell-shaped curve where the light is brightest at the center, with a gradual drop-off in intensity on either side. The arrangement of the nanopillars affects the intensity, creating a beam with uniform brightness, which increases the efficiency of its use.

The beam — now uniform and properly polarized (polarization as well is a critical factor in laser cooling) — hits a diffraction that splits the beam into three pairs of equal and oppositely directed beams. Combined with an applied magnetic field, the four beams, pushing on the atoms in opposing directions, serve to trap the cooled atoms.

Each component of the system — the converter, metasurface, and grating — had been developed at NIST, though in operation at separate laboratories on the two NIST campuses.

“That's the fun part of this story,” McGehee said. “I knew all the NIST scientists who had independently worked on these different components, and I realized the elements could be put together to create a miniaturized laser cooling system.”

Though the system will have to be 10 times smaller to cool atoms on a chip, the experiment “is proof of principle that it can be done,” McGehee said. “Ultimately, making the light preparation smaller and less complicated will enable laser-cooling-based technologies to exist outside of laboratories.”

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, contributed to the study.

The research was published in the New Journal of Physics (www.doi.org/10.1088/1367-2630/abdce3).