Scientists Develop Small Lens for Trapping Atoms

Atoms are notoriously tough to regulate. They move in a zigzag pattern similar to fireflies, can escape from the most durable containers, and even exhibit random movements at temperatures close to absolute zero.

For quantum tools like atomic clocks or quantum computers to function properly, researchers must be able to capture and manipulate individual atoms. If individual atoms can be contained and manipulated over large distances, they have the potential to function as quantum bits or qubits. These tiny units of information can use the state or orientation of the atom to perform calculations at speeds much faster than even the most powerful supercomputers.

A team of researchers from the National Institute of Standards and Technology (NIST), in collaboration with partners from JILA, a joint institute of the University of Colorado and NIST located in Boulder, have successfully demonstrated a novel miniaturized version of “optical tweezers” for capturing individual atoms using a laser beam as “chopsticks”. This marks the first instance of single-atom trapping using this method.

Typically, optical tweezers, which were awarded the 2018 Nobel Prize in Physics, involve the use of large lenses that are several centimeters in size or microscopic lenses placed outside a vacuum chamber for trapping individual atoms. In the past, NIST and JILA have utilized this method to develop an atomic clock with great success.

A Recent Design

In the recent design, instead of conventional lenses, the NIST team utilized non-traditional optics consisting of a square glass wafer, measuring approximately 4 millimeters in length, that is etched with numerous pillars each just a few hundred nanometers in height. These pillars, collectively known as metasurfaces, function as miniature lenses. They can focus laser light to capture, manipulate and image individual atoms within a vapor.

Unlike regular optical tweezers, metasurfaces can function within the vacuum chamber where a cloud of trapped atoms is present.

The process entails several steps. At first, when an uncomplicated form of light, called an “airplane wave”, comes into contact with clusters of small nanopillars, the nanopillars alter the plane wave into a sequence of minor waves, each slightly out of phase with the one beside it. This causes the adjacent waves to reach their maximum point at different times.

The wavelets then interact or “interfere” with one another, causing them to concentrate their energy at a specific point, which is where the targeted atom will be trapped. By adjusting the angle of the incoming plane waves of light that hit the nanopillars, the wavelets are focused on slightly different positions, allowing the optical system to capture multiple atoms located in slightly different areas from each other.

According to Amit Agrawal, a scientist at NIST, the use of these small, flat lenses in a vacuum chamber eliminates the need for a complex optical system with moving parts to trap atoms.

Previous to this research, scientists at both NIST and JILA had effectively employed conventional optical tweezers to design atomic clocks.

In the new research, Agrawal and two other NIST scientists, Scott Papp and Wenqi Zhu, and partners from Cindy Regal’s group at JILA, designed, produced, and evaluated the metasurfaces and did single-atom capturing experiments.

New Lens Technique

The researchers published a paper in PRX Quantum today stating that they had successfully trapped 9 individual rubidium atoms using this method. Agrawal believes that scaling up the technique by using multiple metasurfaces or a larger field of view could lead to the trapping of multiple single atoms and pave the way for a chip-scale optical system to routinely trap a variety of atoms.

The system held the atoms in position for approximately 10 secs, which is long enough to research the quantum mechanical homes of the particles and also use them to save quantum details. In quantum experiments, timeframes of microseconds to milliseconds are common. The scientists illuminated the trapped rubidium atoms with a separate light source to verify their capture by inducing them to fluoresce. The metasurfaces had previously played a crucial role in forming and focusing the incoming light that trapped the rubidium atoms, and now they played another critical role. They captured and focused the fluorescent light emitted by the same atoms, redirecting the fluorescent radiation to a camera to capture images of the atoms.

The metasurfaces have capabilities beyond just trapping individual atoms. They can use their precise light-focusing abilities to manipulate individual atoms into specific quantum states tailored for specific experiments involving atom trapping.

The tiny lenses can use polarized light to orient an atom’s spin in a particular direction, much like the rotation of the Earth on its axis. These interactions between focused light and individual atoms are valuable for various atom-scale experiments and devices, including the development of future quantum computers.

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

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