“Optical tweezers— systems that focus light to capture and manipulate individual atoms — can pave the way for powerful quantum devices, but they can be a bit cumbersome. Researchers have now developed a simplified, smaller design for an optical tweezer that uses a metasurface lens studded with millions of tiny pillars.
Given their small size, individual atoms are notoriously tricky to see and manipulate, but it would be extremely helpful to find ways to do this. The invention of the laser in the 1960s eventually led to the realization that the radiant pressure of light could be used to trap particles, atoms and even living bacteria. In the 1980s, the optical tweezers were born, with which their makers 2018 Nobel Prize in Physics.
As powerful as these “tools of light” have been, they require relatively large centimeter-scale lenses and image the atoms with separate microscope systems that cannot operate in the vacuum where the atoms were originally held and trapped. But for the new study, scientists at the National Institute of Standards and Technology (NIST) and JILA developed a new type of optical tweezers that solve both problems.
The new design uses a 4mm square of glass etched with tiny silicon pillars each several hundred nanometers high. This forms a meta-surface that fine-tunes the incoming laser light and focuses it on a cloud of atoms in the vacuum, picking one to capture.
The system works in a pretty smart way. First, the laser light is emitted as a plane wave, meaning it propagates as a series of flat plates. When these plates hit the meta-surface, the nanopillars transform the light waves into smaller “waves” that are slightly out of sync with each other, so they reach their peaks at different times. This structure causes the wavelets to interfere with each other and effectively concentrate all their energy on a very fine point – and the atom located at that point will get stuck.
Hitting the meta-surface with plane waves coming from different angles can focus wavelets at different points, allowing the tweezers to capture multiple individual atoms at once. Unlike existing systems, this can be done directly in the vacuum chamber where the target atoms are stored and no moving parts are required.
In tests, the team demonstrated the meta-surface by individually trapping nine rubidium atoms and holding each for about 10 seconds. The researchers tracked the trapped atoms by hitting them with a separate light source causing them to fluoresce, demonstrating yet another advantage of their new system: the metasurface can essentially also work in reverse, capturing the fluorescence emitted by the atoms and in an external camera to image the atoms.
The researchers say the new system could be scaled up with a larger field of view or multiple meta-surfaces working simultaneously, potentially allowing them to capture and manipulate hundreds of atoms at once. This could form the basis of quantum computer memory, where data is processed and stored in the quantum states of each atom.
The research is published in the journal PRX Quantum.