Researchers develop miniature lens to capture atoms

Atoms are notoriously difficult to control. They zigzag like fireflies, tunnel out of the strongest containers and tremble even at temperatures close to absolute zero.

Still, scientists need to capture and manipulate individual atoms in order for quantum devices like atomic clocks or quantum computers to work properly. If individual atoms can be linked together and controlled in large arrays, they can serve as quantum bits, or qubits — tiny discrete units of information whose state or orientation can eventually be used to perform calculations at speeds well in excess of the fastest supercomputer.

Researchers from the National Institute of Standards and Technology (NIST), along with collaborators from JILA – a joint institute of the University of Colorado and NIST in Boulder – have shown for the first time that they can trap single atoms with a novel miniaturized version of “optical tweezers.” ‘ – a system that uses a laser beam to grasp atoms as chopsticks.

Typically, optical tweezers, which won the 2018 Nobel Prize in Physics, feature bulky centimeter-sized lenses or microscopic lenses outside of the vacuum that hold single atoms. NIST and JILA have previously used the technique to produce an atomic clock with great success.

In the new design, instead of typical lenses, the NIST team used unconventional optics — a square glass wafer about 4 millimeters long printed with millions of columns that are just a few hundred nanometers (billionths of a meter) high and together appear tiny lenses work. These embossed surfaces, called metasurfaces, focus laser light to capture, manipulate, and image individual atoms in a vapor. Unlike ordinary optical tweezers, the metasurfaces can work in the vacuum in which the cloud of trapped atoms resides.

The process involves several steps. First, incident light, which has a particularly simple shape called a plane wave, hits groups of the tiny nanopillars. (Plane waves are like moving parallel sheets of light with a uniform wavefront, or phase, whose vibrations stay in sync with each other, neither diverging nor converging along their path.) The arrays of nanopillars transform the plane waves into a series of small wavelets, each of which is slightly is asynchronous with its neighbor. As a result, adjacent wavelets peak at slightly different times.

These wavelets combine, or “interfere,” with each other, causing them to focus all of their energy on a specific location – the location of the atom to be captured.

Depending on the angle at which the incident plane light waves strike the nanopillars, the wavelets are focused at slightly different locations, allowing the optical system to capture a series of individual atoms located at slightly different locations from each other.

Because the mini-flat lenses can operate in a vacuum chamber and require no moving parts, the atoms can be captured without the need to build and manipulate a complex optical system, said NIST researcher Amit Agrawal. Other NIST and JILA researchers have previously used conventional optical tweezers to design atomic clocks with great success.

In the new study, Agrawal and two other NIST scientists, Scott Papp and Wenqi Zhu, along with collaborators from Cindy Regal’s group at JILA, designed, fabricated and tested the metasurfaces and performed single-atom capture experiments.

In an article published today in PRX Quantum, the researchers reported that they had captured nine individual rubidium atoms separately. The same technique, magnified by using multiple metasurfaces or one with a wide field of view, should be able to trap hundreds of individual atoms, Agrawal said, and could lead the way to routinely image an array of atoms using a chip-scale optical system to capture .

The system held the atoms in place for about 10 seconds, long enough to study the particles’ quantum mechanical properties and use them to store quantum information. (Quantum experiments operate on time scales from tens of millionths to thousandths of a second.)

To demonstrate that they captured the rubidium atoms, the researchers illuminated them with a separate light source, causing them to fluoresce. The metasurfaces then played a second crucial role. First, they shaped and focused the incident light that captured the rubidium atoms. Now the metasurfaces captured and focused the fluorescent light emitted by the same atoms and redirected the fluorescent radiation into a camera to image the atoms.

The metasurfaces can do more than just enclose individual atoms. By focusing the light with pinpoint accuracy, the metasurfaces can place individual atoms into special quantum states that are tailor-made for specific atom-trapping experiments.

For example, polarized light directed by the tiny lenses can cause an atom’s spin – a quantum attribute that corresponds to the Earth’s rotation on its own axis – to point in a certain direction. These interactions between focused light and individual atoms are useful for many types of atomic-scale experiments and devices, including future quantum computers.

This story is republished with permission from NIST. Read the original story here.