Scientists at the National Institute of Standards and Technology (NIST) and JILA, formerly the Joint Institute for Laboratory Astrophysics, developed a miniaturized optical tweezer that captured single atoms. The challenging task of single-atom captures has implications in quantum technologies, for which single atoms can provide a platform. A means to trap and manipulate single atoms is required for the operation of quantum devices such as atomic clocks and quantum computers. If individual atoms can be collected and controlled in large arrays, they can serve as qubits, which could enable quantum computers to perform calculations at far greater speeds than today’s fastest supercomputer. The researchers used the nanosize tweezer to separately trap nine single rubidium atoms. According to the team, the same technique, scaled up, should be usable to confine hundreds of single atoms. The technique could lead to the development of an optical system for the routine trapping of atom arrays. Instead of a conventional lens, the researchers used a high-numerical aperture, dielectric metasurface lens to build the miniaturized tweezer. They imprinted a square glass wafer, about 4 mm long, with millions of metasurfaces, to act as tiny lenses. Light in the form of plane waves was shined on groupings of nanopillars. The nanopillars converted the plane waves into a series of wavelets, each one slightly out of synch with its neighbor. As a result, adjacent wavelets reached their peak at different times. When the wavelets interfered, the interference caused each wavelet to focus all its energy on a single atom in a specific location. The angle at which the incoming plane waves hit the nanopillars directed the focus of the wavelets, causing each wavelet to focus on a different spot. This allowed the researchers to use an optical system to trap a series of individual atoms, each located at a slightly different spot. Light focusing using a planar glass surface studded with millions of nanopillars, the metalens, forming an optical tweezer. (A) Device cross section depicts plane waves of light that achieve focus through secondary wavelets generated by nanopillars of varying size. (B) The same metalens is used to trap and image single rubidium atoms. Courtesy of Sean Kelley/NIST. Unlike a traditional lens, the metalens was small enough to operate inside the vacuum that held the vapor cloud of atoms. Conventional optical tweezers have bulky, centimeter-size lenses or microscope objectives that are positioned outside the vacuum. The metalens could be used within the vacuum chamber, and it required no moving parts, enabling the atoms in the vacuum to be trapped without the need for a complex optical system, said researcher Amit Agrawal. The researchers formed an atom array by combining the metasurface lens with tunable acousto-optic deflectors. They tested the metasurfaces and performed single-atom trapping experiments that showed that the metalens could trap and image single atoms with a long working distance of 3 mm from the lens. In tests, the miniature optical tweezer held the atoms in place for about 10 s, which is long enough to study the quantum mechanical properties of atoms and use the atoms to store quantum information. Quantum experiments typically operate on timescales of ten millionths to thousandths of a second. To demonstrate the successful capture of nine single rubidium atoms, the researchers illuminated the atoms with a separate light source. The metasurfaces that shaped and focused incoming light to trap the atoms now captured and focused the fluorescent light emitted by the atoms. The metasurfaces then redirected the fluorescent radiation into a camera so that the irradiated atoms could be imaged. The team ultimately demonstrated single-atom trapping with a measured numerical aperture of 0.55. In addition to atom-trapping, the metasurfaces can be used to coax individual atoms into special quantum states, tailored for specific atom-trapping experiments. For example, polarized light directed by the metalens can cause an atom’s spin to point in a certain direction. Light-matter interactions such as these could be useful for atomic-scale experiments and devices, including quantum computers. The NIST team predicted that metasurfaces for atom trapping will be able to build on future developments in metasurface design to allow multifunctional control in complex quantum experiments with atom arrays. The research was published in PRX Quantum (www.doi.org/10.1103/PRXQuantum.3.030316).