Trapping and optically interfacing laser-cooled neutral atoms are essential requirements for their use in advanced quantum technologies. Here we simultaneously realize both of these tasks with cesium atoms interacting with a multicolor evanescent field surrounding an optical nanofiber. The atoms are localized in a one-dimensional optical lattice about 200 nm above the nanofiber surface and can be efficiently interrogated with a resonant light field sent through the nanofiber. Our technique opens the route towards the direct integration of laser-cooled atomic ensembles within fiber networks, an important prerequisite for large scale quantum communication schemes. Moreover, it is ideally suited to the realization of hybrid quantum systems that combine atoms with, e.g., solid state quantum devices.

The strong evanescent field around ultrathin unclad optical fibers bears a high potential for detecting, trapping, and manipulating cold atoms. Introducing such a fiber into a cold-atom cloud, we investigate the interaction of a small number of cold cesium atoms with the guided fiber mode and with the fiber surface. Using high resolution spectroscopy, we observe and analyze light-induced dipole forces, van der Waals interaction, and a significant enhancement of the spontaneous emission rate of the atoms. The latter can be assigned to the modification of the vacuum modes by the fiber.

The guided modes of sub-wavelength diameter air-clad optical fibers exhibit a pronounced evanescent field. The absorption of particles on the fiber surface is therefore readily detected via the fiber transmission. We show that the resulting absorption for a given surface coverage can be orders of magnitude higher than for conventional surface spectroscopy. As a demonstration, we present measurements on sub-monolayers of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) molecules at ambient conditions, revealing the agglomeration dynamics on a second to minutes timescale.PACS numbers: 78.66. Qn, 39.30.+w, 68.43.Jk, 78.66.Jg During the last twenty years, numerous optical tools for surface and interface analysis have been developed [1]. The selective sensitivity to surface effects is often obtained by carrying out spectroscopy with evanescent waves (EW), created by total internal reflection of light at the interface. This is straightforwardly realized by exciting waveguide modes in unclad optical fibers [2,3]. If the EW is resonant with the transition frequency of particles (atoms, molecules, quantum dots, etc.) in the surrounding medium, one can use both the particles' fluorescence [4] or the peak attenuation of the waveguide mode [2,5] to infer the concentration of particles at the interface. Moreover, the line shapes allow to spectroscopically retrieve detailed physical information about the nature and strength of the particle-surface interaction.Fiber-based evanescent wave spectroscopy (EWS) is used in various sensors [6]. The robustness, reliability, and ease of use of an all-fiber-based sensor technology is advantageous for in situ sensing in a remote or isolated location or in a harsh environment, e.g., in industrial applications or environmental studies. Furthermore, such sensors also profit from the multiplexing and miniaturization potential inherent to fiber technology. When measuring a volumetric concentration of particles in the surrounding medium, these sensors yield however a reduced sensitivity compared to conventional free-beam absorption: a significant fraction of the light propagates inside the waveguide and therefore does not interact with the particles of interest. This problem can partially be overcome by increasing the power fraction in the EW through proper choice of the fiber mode or geometry [7,8,9]. Yet, even in the ultimate case of 100 % EW, the sensitivity will not exceed that of free-beam absorption techniques.In this letter, we demonstrate that the situation can be dramatically different when employing fiber-based EWS for the spectroscopic study of surface coverages instead of volumetric concentrations: The ultimate sensitivity of fiber-based surface absorption spectroscopy (SAS) is shown to strongly depend on the fiber diameter and to exceed free-beam SAS by several orders of magnitude in the case of sub-wavelength diameter fibers. Fiber-based surface absorption spectroscopy (SAS) has already been used for a number of applications, e.g., in bio-sensors [10]. However, to our kno...

We dispersively interface an ensemble of one thousand atoms trapped in the evanescent field surrounding a tapered optical nanofiber. This method relies on the azimuthally-asymmetric coupling of the ensemble with the evanescent field of an off-resonant probe beam, transmitted through the nanofiber. The resulting birefringence and dispersion are significant; we observe a phase shift per atom of ∼ 1 mrad at a detuning of six times the natural linewidth, corresponding to an effective resonant optical density per atom of 0.027. Moreover, we utilize this strong dispersion to nondestructively determine the number of atoms.PACS numbers: 42.50. Ct, 37.10.Gh, 37.10.Jk We have recently demonstrated a new technique for trapping and optically interfacing cold atoms [1]. Our method employs one-dimensional arrays of laser-cooled atoms trapped in a two-color evanescent field surrounding an optical nanofiber. The resulting atomic ensemble is both well-isolated from perturbations by the environment and efficiently coupled to a fiber-guided probe field. This makes our system a prime candidate for interfacing and manipulating trapped atoms with light.In [1], the detection of cesium atoms was achieved by monitoring the transmission of resonant probe light through the nanofiber. This probe light couples efficiently to the atoms via its evanescent field resulting in an absorbance per atom of the order of one percent. This strong absorbance also implies that there is a significant phase shift of the probe light in the dispersive regime. In this paper, we present experimental evidence of this phase shift and show that it leads to a frequencydependent birefringence that acts on the polarization state of the probe light propagating through the fiber.Being based on dispersive detection, our method has significant advantages over absorption or fluorescencebased techniques [2]. As an example, its signal-to-noise ratio is superior in the case of high optical depth when assuming shot-noise-limited detection [3]. Conceptually, it is similar to other dispersive detection schemes for atoms and molecules such as interferometry [4,5], frequency modulation spectroscopy [6], or phase-contrast imaging [7].In all these approaches, the phase shift induced by the atomic medium on the probe beam is compared to the phase of a reference beam via interference. In the case of atoms trapped using a nanofiber, this can be accomplished by interfering two orthogonal polarization modes, which couple unequally to the atomic ensemble. The polarization state of the output light thus enables one to infer the phase shift caused by the atoms. Figure 1 shows a schematic of the experimental setup. The atoms are trapped in two one-dimensional arrays FIG. 1. Schematic of the setup: An off-resonant laser beam is coupled into the nanofiber to probe the cesium atoms, which are trapped in the evanescent field of the nanofiber forming two one-dimensional arrays above and below the fiber (zoomed inset). A Stokes measurement is performed on the outgoing probe beam using a quart...

We present experimental techniques and results related to the optimization and characterization of our nanofiberbased atom trap [Vetsch et al., Phys. Rev. Lett. 104, 203603 (2010)]. The atoms are confined in an optical lattice which is created using a two-color evanescent field surrounding the optical nanofiber. For this purpose, the polarization state of the trapping light fields has to be properly adjusted. We demonstrate that this can be accomplished by analyzing the light scattered by the nanofiber. Furthermore, we show that loading the nanofiber trap from a magneto-optical trap leads to sub-Doppler temperatures of the trapped atomic ensemble and yields a sub-Poissonian distribution of the number of trapped atoms per trapping site.Index Terms-Nanophotonics, optical nanofibers, laser cooling and trapping of atoms. 2012 c IEEE, see http://www.ieee.org for copyright policy IEEE

We demonstrate optical transport of cold cesium atoms over millimeter-scale distances along an optical nanofiber. The atoms are trapped in a onedimensional optical lattice formed by a two-color evanescent field surrounding the nanofiber, far red-and bluedetuned with respect to the atomic transition. The bluedetuned field is a propagating nanofiber-guided mode while the red-detuned field is a standing-wave mode which leads to the periodic axial confinement of the atoms. Here, this standing wave is used for transporting the atoms along the nanofiber by mutually detuning the two counter-propagating fields which form the standing wave. The performance and limitations of the nanofiber-based transport are evaluated and possible applications are discussed.

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