“Optical tweezers” – systems that focus light to trap and manipulate individual atoms – could pave the way for powerful quantum devices, but they can be a bit bulky. The researchers have now developed a simplified, smaller design for the optical tweezers that uses a metasurface lens studded with millions of tiny pillars.
Given their small size, individual atoms are notoriously difficult to see and manipulate, but finding ways to do this would be extremely helpful. The invention of the laser in the 1960s eventually led to the realization that the radiation pressure of light could be harnessed to trap particles, atoms and even living bacteria. In the 1980s, optical tweezers were born, earning their creators the 2018 Nobel Prize in Physics.
As powerful as these “tools made of light” are, they require relatively large centimeter lenses and image atoms using separate microscope systems that cannot operate in a vacuum where atoms are originally kept and trapped. But for the new study, scientists from the National Institute of Standards and Technology (NIST) and JILA developed a new type of optical tweezer that solves both problems.
The new design uses a 4mm (0.2 inch) square of glass, etched with tiny pillars of silicon each measuring a few hundred nanometers high. This forms a metasurface that fine tunes incoming laser light and focuses it onto a cloud of atoms in a vacuum, picking one to trap.
The system works quite intelligently. First, the laser light is emitted as a plane wave, meaning it travels as a series of flat sheets. When these sheets hit the metasurface, the nanopillars transform the light waves into smaller “wavelets”, slightly offset from each other, so that they reach their peaks at different times. This structure causes the wavelets to interfere with each other and effectively concentrate all their energy at a very fine point – and the atom at that point will be trapped.
By hitting the metasurface with plane waves from different angles, the wavelets can be focused on different points, allowing the tweezers to trap multiple individual atoms simultaneously. Unlike existing systems, this can be done directly inside the vacuum chamber where the target atoms are kept, and requires no moving parts.
In tests, the team demonstrated the metasurface by separately trapping nine rubidium atoms, holding them each for about 10 seconds. The researchers tracked the trapped atoms by hitting them with a separate light source which made them fluoresce, and which showed another advantage of their new system: the metasurface can also work in reverse, collecting the fluorescence emitted by the atoms. and directing it into an external camera to image the atoms.
The researchers say the new system could be extended with a wider field of view or multiple metasurfaces working in unison, 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 was published in the journal Quantum PRX.