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Focusing through coupled multimode fibers for miniaturized single-atom tweezer arrays

Benjamin Maddox, Daniel K. Ruttley

Published Oct 30, 2026• 0 citations
Publisher

Abstract

Real-world deployment of quantum technologies requires systems in low SWaP-C packages. Quantum platforms based on ultracold atoms typically trap and control atoms with optical tweezers using light focused through bulky objective lenses, which are costly and sensitive to misalignment. Multimode fibers (MMFs) offer a compact route to focusing light, but their use in forming optical tweezers for cold atoms has been limited to a single fiber. Here we use a 2x1 MMF fused coupler to obtain two independent control paths sharing a common distal facet. We form near-diffraction-limited tweezers through each channel, suppress interference by applying a small frequency offset between the channels, and demonstrate that the tweezer separation is insensitive to misalignment of the input coupling. This work provides a route to reconfigurable single-atom arrays in low SWaP-C environments.

DOI
10.5281/zenodo.23799461

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Real-world deployment of quantum technologies requires systems in low SWaP-C packages. Quantum platforms based on ultracold atoms typically trap and control atoms with optical tweezers using light focused through bulky objective lenses, which are costly and sensitive to misalignment. Multimode fibers (MMFs) offer a compact route to focusing light, but their use in forming optical tweezers for cold atoms has been limited to a single fiber. Here we use a 2x1 MMF fused coupler to obtain two independent control paths sharing a common distal facet. We form near-diffraction-limited tweezers through each channel, suppress interference by applying a small frequency offset between the channels, and demonstrate that the tweezer separation is insensitive to misalignment of the input coupling. This work provides a route to reconfigurable single-atom arrays in low SWaP-C environments.
Real-world deployment of quantum technologies requires systems in low SWaP-C packages. Quantum platforms based on ultracold atoms typically trap and control atoms with optical tweezers using light focused through bulky objective lenses, which are costly and sensitive to misalignment. Multimode fibers (MMFs) offer a compact route to focusing light, but their use in forming optical tweezers for cold atoms has been limited to a single fiber. Here we use a 2x1 MMF fused coupler to obtain two independent control paths sharing a common distal facet. We form near-diffraction-limited tweezers through each channel, suppress interference by applying a small frequency offset between the channels, and demonstrate that the tweezer separation is insensitive to misalignment of the input coupling. This work provides a route to reconfigurable single-atom arrays in low SWaP-C environments.
A high imaging resolution or a tight focus of a laser beam imposes a certain minimal numerical aperture (NA) of the optical system. Macroscopic high NA lenses constraint strongly the spatial organization of experiments, in particular when imaging or trapping cold atoms inside a vacuum chamber. Multimode fibers, in conjunction with spatial light modulators, offer an interesting alternative to high NA lenses. Indeed, they are flexible optical waveguides with very small transverse dimensions (∼ 100 μm), and reasonably high NA (up to 0.5). For those reasons, the use of multimode fibers for imaging or laser manipulation purposes has been widely studied in the past years. Here, we transfer these techniques to the field of cold atoms with a multimode fiber forming a compact optical bridge between the inside and the outside of an ultra-high vacuum chamber. We manipulate atoms with laser beams produced through the fiber by digital optical phase conjugation with a spatial light modulator. We are able to transport a small cloud of cold rubidium atoms with a moving optical lattice at about 200 μm from the fiber tip. We can then load them in small optical tweezers with a waist of 1.2 μm. By characterizing the propagation of light modes inside the fiber, we numerically invert its transformation and we reconstruct absorption images of the trapped atoms with a resolution of approximately 1 μm. These results pave the way towards the efficient use of multimode fibers in spatially constrained quantum technology platforms relying on cold atoms.
Abstract Trapping of single ultracold atoms is an important tool for applications ranging from quantum computation and communication to sensing. However, most experimental setups, while very precise and versatile, can only be operated in specialized laboratory environments due to their large size, complexity and high cost. Here, we introduce a new trapping concept for ultracold atoms in optical tweezers based on micrometer-scale lenses that are 3D printed onto the tip of standard optical fibers. The unique properties of these lenses make them suitable for both trapping individual atoms and capturing their fluorescence with high efficiency. In an exploratory experiment, we have established the vacuum compatibility and robustness of the structures, and successfully formed a magneto-optical trap for ultracold atoms in their immediate vicinity. This makes them promising components for portable atomic quantum devices.
Scalability remains a major challenge in building practical fault-tolerant quantum computers. Currently, the largest number of qubits achieved across leading quantum platforms ranges from hundreds to thousands. In atom arrays, scalability is primarily constrained by the capacity to generate large numbers of optical tweezers, and conventional techniques using acousto-optic deflectors or spatial light modulators struggle to produce arrays much beyond ∼10,000\sim 10,000∼10,000 tweezers. Moreover, these methods require additional microscope objectives to focus the light into micrometer-sized spots, which further complicates system integration and scalability. Here, we demonstrate the experimental generation of an optical tweezer array containing 280×280280\times 280280×280 spots using a metasurface, nearly an order of magnitude more than most existing systems. The metasurface leverages a large number of subwavelength phase-control pixels to engineer the wavefront of the incident light, enabling both large-scale tweezer generation and direct focusing into micron-scale spots without the need for a microscope. This result shifts the scalability bottleneck for atom arrays from the tweezer generation hardware to the available laser power. Furthermore, the array shows excellent intensity uniformity exceeding 90%90\%90%, making it suitable for homogeneous single-atom loading and paving the way for trapping arrays of more than 10,00010,00010,000 atoms in the near future.
We demonstrate a miniature, fiber-coupled optical tweezer to trap a single atom. The same fiber is used to trap a single atom and to read out its fluorescence. To obtain a low background level, the tweezer light is chopped, and we measure the influence of the chopping frequency on the atom's lifetime. We use the single atom as a single-photon source at 780 nm and measure the second-order correlation function of the emitted photons. Because of its miniature, robust, fiber-pigtailed design, this tweezer can be implemented in a broad range of experiments where single atoms are used as a resource.
Integrating the scalability of individually addressable arrays of optical-tweezer-trapped single atoms with the efficient light-matter interface provided by nanophotonic waveguides has been a long-standing challenge in quantum technologies based on atoms and photons. Here we realize a quantum interface between photons guided in an optical nanofiber with a diameter of 310 nm and an array of on average 155 individually addressable atoms. Using a spatial light modulator and an objective lens with NA = 0.45, single cesium atoms are trapped in a one-dimensional array of 200 optical tweezer spots with micrometer-scale trap sizes on the nanofiber. Individual atoms are addressed by spatially scanning an excitation laser beam, focused to a spot size comparable to that of the traps through the same objective lens, along the nanofiber. We confirm the single-atom nature of the individual trapping sites through photon-correlation measurements of the guided fluorescence, observing strong photon antibunching with g(2)(0)≈0.26g^{(2)}(0) \approx 0.26g(2)(0)≈0.26. We measure trap lifetimes of a few hundred milliseconds, with a maximum value of 460 ms, at an atom-surface separation of 670 nm without active cooling, representing an order-of-magnitude improvement over previous nanofiber traps. This platform opens a new regime for atom-photon interfaces, paving the way for scalable distributed quantum computing and quantum networks, as well as for the exploration of collective radiative effects in waveguide QED with individually addressable atoms.