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.
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.
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 study of atom-light interaction is a key element of quantum optics and a central part of atomic physics. Systems composed of atoms interacting with each other through the electromagnetic field can be used for studies from fundamental research to practical applications. Experimental realizations of these systems benefit from three distinct attributes: large atom-light coupling, trapping and control of atomic ensembles, and engineering and manipulation of the electromagnetic field. Optical waveguides provide a platform that achieves these three goals. In particular, optical nanofibers are an excellent candidate. They produce a high confinement of the electromagnetic field that improves atom-light coupling, guiding the field that mediates the interactions between atoms, while allowing trapping of the atoms close to it. This thesis describes the uses of an optical nanofiber for quantum optics experiments, demonstrating its possibilities for enabling special atom-light interactions. We trap atoms near the optical nanofiber surface, and characterize the trap in a non-destructive manner. We show how the presence of the nanofiber modifies the fundamental atomic property of spontaneous emission, by altering the electromagnetic environment of the atom. Finally, we use the nanofiber to prepare collective states of atoms around it. These states can radiate faster or slower than a single atom (superradiance and subradiance). The observation of subradiance of a few atoms, a rather elusive effect, evidences nanofibers as a strong candidate for future quantum optics experiments. Moreover, we show how the guided field mediates interaction between atoms hundreds of wavelengths apart, creating macroscopically delocalized collective states.
Fluorescence of atoms has been the subject of interest in quantum optical measurementsfor many years. The atomic fluorescence is routinely used as a tool to detectatoms, to generate photons and to generate non-classical states of light. One of thewidely used methods to investigate the atomic fluorescence is laser-induced fluorescencespectroscopy. This thesis presents the development of optical nanofibers as atool for the measurement of fluorescence spectrum of atoms.Optical nanofibers are ultrathin optical fibers, with diameters less than the propagationwavelength of the fiber, with a silica core and a vacuum clad. Such opticalnanofibers can support only the single propagating guided-mode. The guided-modesof the nanofiber is confined in the transverse direction of the nanofiber and a partof the propagating field lies outside the fiber, in the evanescent region of the fiber.The mode density of the electromagnetic field is modified by the nanofiber. Due tothe modification of mode density, the spontaneous emission of atoms is modified bythe optical nanofibers and the confinement of the guided-modes results in a strongchanneling of spontaneous emission of atoms into the guided-modes of the nanofiber.Such a property of nanofibers can be used to study fluorescence of atoms around thenanofiber.Optical nanofibers are prepared by the adiabatic tapering of single-mode opticalfibers. Our experiments are performed by overlapping cold Cesium atoms, from amagneto-optical trap (MOT), with the optical nanofiber and observing the fluorescencethrough the guided-modes of the nanofiber. The nanofiber suppresses with highprobability any coupling of scattering light from the excitation laser into the guidedmodesand high coupling efficiency of fluorescence into the guided-modes enables us tomeasure the fluorescence spectrum of atoms present around the nanofiber with a goodsignal-to-noise ratio. In this thesis, we demonstrate the measurement of laser inducedfluorescence (LIF) emission spectra of strongly driven atoms using optical nanofibers.For the fluorescence emission spectrum measurements, we develop and demonstrate anew sensitive and high resolution method consisting of optical heterodyne techniqueand photon correlation spectroscopy. The key point of the method is to extract theinformation of first-order field correlation function from the measured second-orderintensity correlation function. The Fourier transform of the field correlation functionthen gives the fluorescence emission spectrum.The atoms emitting fluorescence in the guided-modes of the nanofiber lie very closeto the nanofiber surface and are subjected to atom-surface interactions. It has beenreported in earlier experiments on nanofiber, that such surface interactions can bemodified by irradiating the MOT-atoms overlapped with the nanofiber with UV lasers.After such irradiations, MOT-atoms are kept from falling into the surface-potentialand behave almost like free-space atoms around the nanofiber. We first perform ourfluorescence spectra measurements under this free-atom conditions. The observedexcitation spectrum lineshape shows a free-atom like Lorentzian behavior, with avery small tail in the red-detuned side (red-tail). The red-tail has been attributedto photoassociation process, due to atoms making transition from free ground statesto bound excited states of the surface potential. Under this conditions, the emissionspectra measured for on-resonant and off-resonant excitations show the well-knownthree-peak Mollow-triplet spectrum of free-space two-level atoms. The observationconfirms that the near surface atoms behave like free-space atoms. Also, the quantitativeagreement between the observed spectra with the theoretical calculationsdemonstrates the validity of the optical nanofiber method for measuring fluorescenceemission spectrum of atoms.As mentioned above, since we are observing near-surface atoms, fluorescence measurementsbased on optical nanofibers may become a unique tool to probe atomsurfaceinteractions. In earlier experiments using nanofiber, signatures of atomsurfaceinteraction have been observed in the excitation spectrum of atoms, in theform of a broad red-tail. The lineshape of the excitation spectrum is understood as aresult of a process where atoms approaching the nanofiber surface fall into the shallowsurface potential, forming long-range atom-surface bound states. In this thesis,we have systematically investigated the evolution of the lineshape of the excitationspectrum from a Lorentzian shape to a broad red-tail condition. The evolution of thespectrum lineshape has been attributed to the changing nanofiber surface condition.The spectrum shape is found to depend on the flux of background Cs-atoms. Wespeculate that this might be due to the adsorption of hot background atoms ontothe nanofiber surface, which changes the surface condition of the nanofiber locallyand helps in loading of cold MOT-atoms into the shallow surface potential. We havefound that the effect of UV laser irradiation is to photoionize the MOT-atoms, andthe generated photoelectrons might help in the desorption of adsorbed atoms on thesurface. We discuss the roles of adsorption and desorption of atoms on the surfaceconditions. We have shown that such changing surface condition can be controlledby adjusting the hot background atom flux and the UV laser intensity. Under suchcontrolled surface condition, we have measured the emission spectrum. The observedemission spectrum under the broad red-tail condition can be explained only by includingthe surface interactions. We also discuss qualitatively the phonon-mode densityfor nanofiber at room temperature and the phonon-mediated decay of atoms in thesurface potential.This work may open up the way for using optical nanofibers as a tool to investigatethe laser-induced fluorescence spectrum of strongly driven atoms, and to study atomsurfaceinteractions. In the future, we hope to create optical dipole traps around thenanofibers by introducing trap laser lights through the guided-modes and performfluorescence measurements of these trapped atoms.
Optical nanofibers – very thin, tapered optical fibers where the waist diameter is less than the propagating light wavelength – have been shown to be very useful tools for atom-light interactions. Their small size and relative ease of integration into optical fiber-based experimental setups, in addition to their minimal perturbation on magneto-optically trapped cold atoms, have ensured their adoption into cold atom physics. Here, we will discuss some recent applications of optical nanofibers to manipulate, trap, and control cold 87Rb atoms in ground or Rydberg states. We will present some recent experimental and theoretical results related to the interactions between the atoms and the optical nanofiber field and introduce some of the limitations observed.