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Red laser in the dark

Optical cooling kit miniaturised in step towards cold atoms on a chip

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Scientists at the National Institute of Standards and Technology (NIST) have miniaturised the optical components for cooling atoms to near absolute zero. This is a significant step towards employing cold atoms on microchips.

At room temperature, atoms are constantly in motion, zooming around at approximately the speed of sound. Cooling the atoms – reducing their kinetic energy – causes them to slow to a crawl and fall to the lowest quantum state, displaying unusual properties. When atoms are cooled to around 0.1m/s, researchers can measure the energy transitions of particles and other quantum properties accurately enough to use for reference in navigation and other applications.

In 1995, US scientists cooled atoms to billionths of a degree above absolute zero for the first time, precooling a gas through laser cooling (carrying away energy with scattered light at carefully selected frequencies) and evaporative cooling (ejecting the warmest atoms from a magnetic trap).

Preparing laser light such that it has the properties for laser cooling typically requires an optical assembly as big as a dinner table, limiting the use of ultracold atoms outside of laboratories.

Now, NIST researcher Dr William McGehee and his colleagues have created a compact optical platform just 15cm long which can cool and trap atoms in a gas in a 1cm-wide region. This is the first miniature cooling system which relies solely on planar optics, rendering it easier to mass produce. Although the system is around 10 times too large to fit on a microchip, it is an important step towards incorporating ultracold atoms into chip-based devices outside the laboratory.

“This is important as it demonstrates a pathway for making real devices and not just small versions of laboratory experiments,” said McGehee.

First, light is launched from an optical integrated circuit, using a device known as an extreme mode convertor to enlarge the laser beam to hundreds of times its original width. The light strikes an ultrathin film studded with tiny pillars 600nm in length and 100nm wide (an example of a metasurface). These tiny pillars widen the laser beam further and alter the intensity and polarisation of the light waves to create a beam with uniform brightness; these changes allowing it to efficiently cool a large number of atoms.

The reshaped beam then strikes a diffraction grating, which splits it into three pairs of equal and oppositely-directed beams. Combined with an applied magnetic field, these beams push on atoms from opposite directions, trapping them in place.

While each component of the system (the convertor, metasurface, and grating) had been developed at NIST, they were in operation at different laboratories. McGehee and his team brought them together for the first time to create the optical system.

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

He says that this is proof of principle that laser-cooled atoms could be incorporated into chips: “Ultimately, making the light preparation smaller and less complicated will enable laser-cooling based technologies to exist outside of laboratories,” he said.

Ultracold atoms on chips could drive a new generation of super-precise atomic clocks and magnetometers, enable navigation without GPS, and simulate quantum systems, which would be impossible to simulate with the most powerful supercomputers.

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