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arXiv:1712.05749 · Physical Review X

Near-Ground-State Cooling of Atoms Optically Trapped 300 nm Away from a Hot Surface

Y. Meng, Alexandre Dareau, Philipp Schneeweiß, Arno Rauschenbeutel

Published Sep 4, 2018• 53 citations
arXivHTMLPDF

Abstract

Laser-cooled atoms coupled to nanophotonic structures constitute a powerful research platform for the exploration of new regimes of light-matter interaction.While the initialization of the atomic internal degrees of freedom in these systems has been achieved, a full preparation of the atomic quantum state also requires controlling the center-of-mass motion of the atoms at the quantum level.Obtaining such control is not straightforward because of the close vicinity of the atoms to the photonic system at ambient temperature.Here, we demonstrate cooling of individual neutral cesium atoms that are optically interfaced with light in an optical nanofiber, preparing them close to their three-dimensional motional ground state.The atoms are localized less than 300 nm away from the hot fiber surface.Ground-state preparation is achieved by performing degenerate Raman cooling, and the atomic temperature is inferred from the analysis of heterodyne fluorescence spectroscopy signals.Our cooling method can be implemented either with externally applied or guided light fields.Moreover, it relies on polarization gradients, which naturally occur for strongly confined guided optical fields.Thus, this method can be implemented in any trap based on nanophotonic structures.Our results provide an ideal starting point for the study of novel effects such as light-induced self-organization, the measurement of novel optical forces, and the investigation of heat transfer at the nanoscale using quantum probes.

DOI
10.1103/physrevx.8.031054

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Laser-cooled atoms coupled to nanophotonic structures constitute a powerful research platform for the exploration of new regimes of light-matter interaction.While the initialization of the atomic internal degrees of freedom in these systems has been achieved, a full preparation of the atomic quantum state also requires controlling the center-of-mass motion of the atoms at the quantum level.Obtaining such control is not straightforward because of the close vicinity of the atoms to the photonic system at ambient temperature.Here, we demonstrate cooling of individual neutral cesium atoms that are optically interfaced with light in an optical nanofiber, preparing them close to their three-dimensional motional ground state.The atoms are localized less than 300 nm away from the hot fiber surface.Ground-state preparation is achieved by performing degenerate Raman cooling, and the atomic temperature is inferred from the analysis of heterodyne fluorescence spectroscopy signals.Our cooling method can be implemented either with externally applied or guided light fields.Moreover, it relies on polarization gradients, which naturally occur for strongly confined guided optical fields.Thus, this method can be implemented in any trap based on nanophotonic structures.Our results provide an ideal starting point for the study of novel effects such as light-induced self-organization, the measurement of novel optical forces, and the investigation of heat transfer at the nanoscale using quantum probes.
Cold atom-nanophotonic systems constitute a powerful research platform for the exploration of new regimes of light-matter interaction. The guided light in nanophotonic systems are tightly confined, and often used to trap and manipulate cold atoms at subwavelength distance away from the surface. While the initialization of atomic internal degrees of freedom in these systems has been achieved, a full control of the atomic quantum state also requires manipulating and preparing the atomic motional state at the quantum level. In this thesis, we explore and characterize the coupling between motional and spin degrees of freedom in nanofiber-trapped atoms. This coupling originates from the strong polarization gradient which occurs naturally in spatially confined light fields, i.e., the guided light field in a nanophotonic system. We demonstrate that the spin-motion coupling can be utilized to implement degenerate Raman cooling and prepare atoms close to the three-dimensional motional ground state.We obtain mean numbers of motional quanta of nanofiber-trapped atoms using heterodyne fluorescence spectroscopy. Building on this work, we then use the spin-motion coupling for nanofiber-trapped atoms to realize a mechanical analogue of the Dicke model. We infer the energy spectrum of the system from transitions observed in the fluorescence spectrum. We show that our system reaches the ultrastrong coupling regime. Moreover, we demonstrate that the coupling strength can be readily tuned using an additional nanofiber-guided light field. Furthermore, we achieve imaging of single nanofiber-trapped atoms. Taking advantage of degenerate Raman cooling, we cool atoms near the motional ground state while collecting atomscattered light using a camera. We show single atoms can be detected by imaging at an integration time far less than the trapping lifetime. To demonstrate the potential of this technique, we perform two proof of principle experiments. First, we measure the extinction of a nanofiberguided light atom by atom, and verify the Beer-Lambert Law in the few atoms regime. Second, we detect the atom-scattered light that is coupled to the nanofiber. We observe interference of scattered light fields as a function of the distance between two trapped atoms. By controlling atomic motional states and imaging single atoms, our work in this thesis adds to the toolbox for manipulating and detecting cold atoms interfaced to nanophotonic systems, and paves the way for realizing the bottom-up approach to explore new regimes of light-matter interaction atom by atom.
The evanescent field surrounding nanoscale optical waveguides offers an efficient interface between light and mesoscopic ensembles of neutral atoms. However, the thermal motion of trapped atoms, combined with the strong radial gradients of the guided light, leads to a time-modulated coupling between atoms and the light mode, thus giving rise to additional noise and motional dephasing of collective states. Here, we present a dipole force free scheme for coupling of the radial motional states, utilizing the strong intensity gradient of the guided mode and demonstrate all-optical coupling of the cesium hyperfine ground states and motional sideband transitions. We utilize this to prolong the trap lifetime of an atomic ensemble by Raman sideband cooling of the radial motion which, to the best of our knowledge, has not been demonstrated in nano-optical structures previously. This Letter points towards full and independent control of internal and external atomic degrees of freedom using guided light modes only.
Quantum control of atoms at ultrashort distances from surfaces would open a new paradigm in quantum optics and offer a novel tool for the investigation of near-surface physics. Here, we investigate the motional states of atoms that are bound weakly to the surface of a hot optical nanofiber. We theoretically demonstrate that with optimized mechanical properties of the nanofiber these states are quantized despite phonon-induced decoherence. We further show that it is possible to influence their properties with additional nanofiber-guided light fields and suggest heterodyne fluorescence spectroscopy to probe the spectrum of the quantized atomic motion. Extending the optical control of atoms to smaller atom-surface separations could create opportunities for quantum communication and instigate the convergence of surface physics, quantum optics, and the physics of cold atoms.
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.
We have recently demonstrated a new experimental platform for trapping and optically interfacing laser-cooled cesium atoms. The scheme uses a two-color evanescent field surrounding an optical nanofiber to localize the atoms in a one-dimensional optical lattice 200 nm above the nanofiber surface [1]. In order to use this fiber-coupled ensemble of trapped atoms for applications in the context of quantum communication and quantum information processing, non-classical states of the atomic spins have to be prepared and should live long enough to allow one to apply successive quantum gates. The close proximity of the trapped atoms to the nanofiber surface and the strong polarization gradients of nanofiber-guided light fields are potentially important sources of decoherence. Here, we present our latest experimental results on the coherence properties of atomic spins in our nanofiber-based trap. Using a microwave field to drive the clock transition, we determine inhomogeneous and homogeneous dephasing times by Ramsey and spin echo techniques, respectively. Our results constitute the first measurement of the coherence properties of atoms trapped in the vicinity of a nanofiber and represent a fundamental step towards establishing nanofiber-based traps for cold atoms as a building block in a quantum network.
Near-fields around nanophotonic structures and waveguides can be used to optically interface particles ranging from atoms and molecules to microscopic biological and synthetic particles. Due to the strong, non-linear dependence of the near-field coupling strength on the particles' position, a change of the spread of the particles' position will change their mean coupling strength. When the particles are trapped, this position spread depends on their temperature, generally leading to temperature-dependent coupling. Here, we experimentally demonstrate that this effect renders optical probing of trapped particles with near fields an inherently transient process. Specifically, we trap cold atoms in a two-color dipole trap surrounding an optical nanofiber and probe them with the evanescent field of guided, resonant light. The scattering of this probe light heats up the atoms, leading to a decrease of the coupling strength as well as loss of atoms. We observe both effects via a concurrent decrease of the absorption signal. In addition, we demonstrate that the coupling strength can be recovered by cooling the atoms back to their initial temperature. Our findings are relevant for numerous situations where stable coupling of trapped particles to a nanophotonic structure is required.