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arXiv:2012.09073 · Physical Review Applied

High-Resolution Imaging of Cold Atoms through a Multimode Fiber

Nicolas Vitrant, Sébastien Garcia, Kilian Müller, Alexei Ourjoumtsev

Published Jun 17, 2021• 2 citations
arXivHTMLPDF

Abstract

We develop an ultracompact high-resolution imaging system for cold atoms. Its only in-vacuum element is a multimode optical fiber with a diameter of 230 $\ensuremath{\mu}\mathrm{m}$, which simultaneously collects light and guides it out of the vacuum chamber. External adaptive optics allow us to image cold $\mathrm{Rb}$ atoms with approximately 1-$\ensuremath{\mu}\mathrm{m}$ resolution over a $100\ifmmode\times\else\texttimes\fi{}100\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{2}$ field of view. These optics can be easily rearranged to switch between fast absorption imaging and high-sensitivity fluorescence imaging. This system is particularly suited for hybrid quantum-engineering platforms where cold atoms are combined with optical cavities, superconducting circuits, or optomechanical devices restricting optical access.

DOI
10.1103/physrevapplied.15.064047

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We develop an ultracompact high-resolution imaging system for cold atoms. Its only in-vacuum element is a multimode optical fiber with a diameter of 230 \ensuremathμm\ensuremath{\mu}\mathrm{m}\ensuremathμm, which simultaneously collects light and guides it out of the vacuum chamber. External adaptive optics allow us to image cold Rb\mathrm{Rb}Rb atoms with approximately 1-\ensuremathμm\ensuremath{\mu}\mathrm{m}\ensuremathμm resolution over a 100\ifmmode×\else\texttimes\fi100\ensuremathμm2100\ifmmode\times\else\texttimes\fi{}100\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{2}100\ifmmode×\else\texttimes\fi100\ensuremathμm2 field of view. These optics can be easily rearranged to switch between fast absorption imaging and high-sensitivity fluorescence imaging. This system is particularly suited for hybrid quantum-engineering platforms where cold atoms are combined with optical cavities, superconducting circuits, or optomechanical devices restricting optical access.
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.
We show that a standard multimode optical fiber can act as a high-resolution ultra-compact tool to manipulate cold atoms in setups with limited optical access. Spatial light modulators allow us to generate control beams at the in-vacuum fiber end by digital optical phase conjugation. With no additional in-vacuum optics, this system reaches a $ \sim 1\;{\unicode{x00B5}{\rm m}} $∼1µm resolution for a transverse size of only 225 µm. As a demonstration, we use it to optically transport cold atoms towards the in-vacuum fiber end, to load them in optical microtraps, and to re-cool them in optical molasses. This work shows that the rapid progress of optics in complex media opens new, to the best of our knowledge, perspectives for spatially constrained quantum technology platforms combining cold atoms with other optical, electronic, or opto-mechanical systems.
In cold atom experiments, high resolution imaging systems have been used to extract in-situ density information when studying quantum gases, which is one of the hot topics in this field. Such a system is usually called “quantum-gas microscope”. In order to achieve a long working distance and large magnification, high resolution imaging of cold atoms through a vacuum window usually requires a long distance between the atoms and the camera. However, due to space limitation caused by a large number of nearby optical elements, it may be difficult to realize a long imaging system, which is a common case in cold atom experiments. Herein we present an imaging system that can achieve a short distance between the atoms and the image plane with diffraction-limited 1 μm resolution and 50 magnification. The telephoto lens design is adopted to reduce the back focal length and enhance the pointing stability of the imaging lens. The system is optimized at an operating wavelength of 767 nm and corrects aberrations induced by a 5-mm-thick silica vacuum window. At a working distance of 32 mm, a diffraction-limited field of view of 408 μm is obtained. The simulation result shows that by changing the air space between lenses, our design operates across a wide range of window thicknesses (0–15 mm), which makes it robust enough to be used in typical laboratories. This compact imaging system is made from commercial on-shelf Φ2 in (1 in = 2.54 cm) singlets and consists of two components: a microscope objective with a numerical aperture of 0.47 and a telephoto objective with a long effective focal length of 1826 mm. Both are infinitely corrected, allowing the distance between them to be adjusted to insert optical elements for irradiating atoms with laser beams of different wavelengths without affecting the imaging resolution. Taking the manufacturing and assembling tolerances into consideration, the Monte Carlo analyses show that more than 95% of the random samples are diffraction-limited within the field of view. This high success rate ensures that these two objectives can be achieved easily in the experiment. Combined with its performance with other wavelengths (470–1064 nm), this imaging system can be used for imaging different atom species, such as sodium, lithium, and cesium.
Transporting cold atoms between interconnected vacuum chambers is an important technique for increasing the versatility of cold atom setups, particularly for those that couple atoms to photonic devices. In this report, we introduce a method where we are able to image the atoms at all points during transport via moving optical dipole trap. Cooled 87Rb atoms are transported ∼50 cm into an auxiliary vacuum chamber while being monitored with a moving-frame imaging system for which in-situ characterization of the atom transport is demonstrated. Precise positioning of the atoms near photonic devices is also tested across several tapered fibers showing an axial positioning resolution of ∼450 μm.
A quantum gas microscope plays an important role in cold-atom experiments, which provides a high-resolution imaging of the spatial distributions of cold atoms. Here we design, build and calibrate an integrated microscope for quantum gases with all the optical components fixed outside the vacuum chamber. It provides large numerical aperture (NA) of 0.75, as well as good optical access from side for atom loading in cold-atom experiments due to long working distance (7 mm fused silica+6 mm vacuum) of the microscope objective. We make a special design of the vacuum viewport with a T-shape window, to suppress the window flatness distortion introduced by the metal-glass binding process, and protect the high-resolution imaging from distortions due to unflattened window. The achieved Strehl ratio is 0.9204 using scanning-near-field microscopy (SNOM) fiber coupling incoherent light as point light source.
Optical traps, including those used in atomic physics, cold chemistry, and quantum science, are widely used in the research on cold atoms and molecules. Owing to their microscopic structure and excellent operational capability, optical traps have been proposed for cold atom experiments involving complex physical systems, which generally induce violent background scattering. In this study, using a background-free imaging scheme in cavity quantum electrodynamics systems, a cold atomic ensemble was accurately prepared below a fiber cavity and loaded into an optical trap for transfer into the cavity. By satisfying the demanding requirements for the background-free imaging scheme in optical traps, cold atoms in an optical trap were detected with a high signal-to-noise ratio while maintaining atomic loading. The cold atoms were then transferred into the fiber cavity using an optical trap, and the vacuum Rabi splitting was measured, facilitating relevant research on cavity quantum electrodynamics. This method can be extended to related experiments involving cold atoms and molecules in complex physical systems using optical traps.