October 7, 2022

Graphical illustration of light focusing using a flat glass surface studded with millions of nanopillars (referred to as ametaals) forming an optical tweezer. (A) Device cross section showing planar light waves coming to focus by secondary waves generated by different sized nanopillars. (B) The same metals are used to capture and image single rubidium atoms. Credit: Sean Kelley/NIST

Atoms are notoriously difficult to control. They zigzag like fireflies, tunneling out of the strongest containers and vibrating even at temperatures around absolute zero.


Nevertheless, scientists must capture and manipulate individual atoms in order for quantum devices, such as atomic clocks or quantum computers, to work properly. If individual atoms can be corralled and controlled in large arrays, they could serve as quantum bits or qubits — small discrete units of information whose state or orientation could ultimately be used to perform calculations at speeds much higher than the fastest supercomputer.

Researchers at 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 demonstrated for the first time that they can capture single atoms using a new miniaturized version of “optical tweezers” – a system that grabs atoms using a laser beam as chopsticks.

Usually, optical tweezers, which won the Nobel Prize in Physics in 2018, have bulky centimeter-sized lenses or microscope objectives outside the vacuum containing individual atoms. NIST and JILA have previously used the technique with great success to create an atomic clock.

In the new design, the NIST team used unconventional optics instead of typical lenses: a square glass wafer about 4 millimeters long printed with millions of pillars just a few hundred nanometers (billionths of a meter) high that together act as tiny lenses. These printed surfaces, called meta-surfaces, focus laser light to capture, manipulate and image individual atoms in a vapor. The meta-surfaces can operate in the vacuum where the cloud of trapped atoms resides, unlike ordinary optical tweezers.

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The process includes several steps. First, incident light of a particularly simple shape, known as a plane wave, strikes groups of the small nanopillars. (Plane waves are like moving parallel beams of light with a uniform wavefront or phase, whose oscillations stay in sync with each other and do not diverge or converge as they travel.) The nanopillar arrays transform the plane waves into a series of small wavelets, each slightly out of sync with their neighbor. As a result, adjacent wavelets peak at slightly different times.

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

Depending on the angle at which the incoming plane light waves hit the nanopillars, the wavelets are focused in slightly different places, allowing the optical system to capture a series of individual atoms that are in slightly different locations from each other.

Because the mini-flat lenses can be operated in a vacuum chamber and require no moving parts, the atoms can be locked in without having to build and manipulate a complex optical system, said NIST researcher Amit Agrawal. Other researchers at NIST and JILA 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 meta-surfaces and conducted experiments for trapping one atom. .

In an article published today in PRX Quantum, the researchers reported that they had individually captured nine single rubidium atoms. The same technique, scaled up by using multiple meta-surfaces or one with a large field of view, should be able to trap hundreds of individual atoms, Agrawal said, and could pave the way for routinely capturing an array of atoms using a chip-scale optical system. .

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The system held the atoms in place for about 10 seconds, which is long enough to study the quantum mechanical properties of the particles and use them to store quantum information. (Quantum experiments operate on timescales from ten-millionths to one-thousandths of a second.)

To show that they captured the rubidium atoms, the researchers illuminated them with a separate light source, causing them to fluoresce. The meta-surfaces then played a second crucial role. Initially, they had shaped and focused the incoming light that trapped the rubidium atoms. Now the meta-surfaces caught and focused the fluorescent light emitted by the same atoms, redirecting the fluorescent radiation to a camera to image the atoms.

The meta-surfaces can trap more than just individual atoms. By focusing light with pinpoint accuracy, the meta-surfaces can put individual atoms into special quantum states, tailored for specific atom-trapping experiments.

For example, polarized light sent through the tiny lenses can cause an atom’s spin — a quantum attribute analogous to Earth spinning on its axis — pointing in a certain direction. These interactions between focused light and single atoms are useful for many types of experiments and atomic-scale devices, including future quantum computers.


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More information:
T.-W. Hsu et al, Single-Atom Trapping in Metasurface Lens Optical Tweezers, PRX Quantum (2022). DOI: 10.1103/PRXQuantum.3.030316

Provided by National Institute of Standards and Technology

This story has been republished courtesy of NIST. Read the original story here.