“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 little cumbersome. Researchers have now developed a simplified, smaller design for optical tweezers that uses a metasurface lens studded with millions of tiny pillars.
Given their tiny size, individual atoms are notoriously tricky to see and manipulate, but finding ways to do so would be extremely useful. 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 live bacteria. By the 1980s, the optical tweezers were born, earning their creators the 2018 Nobel Prize in Physics.
As powerful as these “tools made of light” have been, they require relatively large centimeter-scale lenses, and image the atoms using separate microscope systems that can’t operate in the vacuum where the atoms are originally kept 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 solves both problems.
The new design uses a 4-mm (0.2-in) square of glass, etched with tiny pillars of silicon that each stand a few hundred nanometers tall. This forms a metasurface that precisely tunes the incoming laser light and focuses it on a cloud of atoms in the vacuum, singling out one to trap.
The system works in a pretty clever way. 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,” which 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 focus all their energy into a very fine point – and the atom that happens to be at that point will become trapped.
By hitting the metasurface with plane waves coming from different angles, wavelets can be focused onto different points, which allows the tweezers to trap several individual atoms simultaneously. Unlike existing systems, this can be done right inside the vacuum chamber where the target atoms are kept, and doesn’t require any moving parts.
In tests, the team demonstrated the metasurface by trapping nine rubidium atoms separately, holding onto each for around 10 seconds. The researchers tracked the trapped atoms by hitting them with a separate light source that made them fluoresce, and that showed off another advantage of their new system: the metasurface can essentially work in reverse too, 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 scaled up with a larger field of view or multiple metasurfaces working in unison, allowing them to potentially 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 PRX Quantum.