In this article we discuss and compare different ways to engineer an interface between ultracold atoms and micro- and nanomechanical oscillators. We start by analyzing a direct mechanical coupling of a single atom or ion to a mechanical oscillator and show that the very different masses of the two systems place a limit on the achievable coupling constant in this scheme. We then discuss several promising strategies for enhancing the coupling: collective enhancement by using a large number of atoms in an optical lattice in free space, coupling schemes based on high-finesse optical cavities, and coupling to atomic internal states. Throughout the manuscript we discuss both theoretical proposals and first experimental implementations.
In this article we discuss and compare different ways to engineer an interface between ultracold atoms and micro- and nanomechanical oscillators. We start by analyzing a direct mechanical coupling of a single atom or ion to a mechanical oscillator and show that the very different masses of the two systems place a limit on the achievable coupling constant in this scheme. We then discuss several promising strategies for enhancing the coupling: collective enhancement by using a large number of atoms in an optical lattice in free space, coupling schemes based on high-finesse optical cavities, and coupling to atomic internal states. Throughout the manuscript we discuss both theoretical proposals and first experimental implementations.
In this thesis I present experiments investigating controlled coupling between mechanical oscillators and ultracold atoms. I report on three different coupling mechanisms. In a first experiment, the surface potential experienced by atoms close to the mechanical oscillator is employed to couple the oscillator motion to the center of mass (COM) motion of a trapped Bose-Einstein condensate (BEC). The magnetic trapping potential is modified by the surface potential arising from the oscillator surface which results in a reduced trap depth. Vibration of the oscillator leads to a modulation of the trap frequency and the minimum of the trapping potential. Observing the loss of atoms from the BEC allows us to read out the amplitude of the mechanical oscillator with the atoms. In a second experiment, we study the coupling of a mechanical membrane oscillator and thermal atoms trapped in a 1D optical lattice. The membrane is the end mirror of the lattice, and oscillation of the membrane couples to the COM mode of the atomic ensemble. Conversely, the center of mass motion of the atomic ensemble redistributes photons between the two running waves forming the 1D optical lattice, effectively modulating their power, and hence the radiation pressure acting onto the membrane. We observe the action of the oscillating membrane onto the atoms by detecting the resulting temperature increase of the atomic ensemble in absorption imaging. To observe the backaction of the atoms onto the mechanical oscillator, the mechanical damping is measured in experiments with and without atoms in the lattice, and we measure higher damping in the presence of atoms in agreement with the theoretical predictions. These experiments are the first demonstration of backaction of an atomic system onto a mechanical oscillator. We investigate a third coupling mechanism, where the motion of a mechanical oscillator is coupled to the collective spin of a BEC. The tip of a mechanical oscillator is functionalized with a magnet, which transduces the oscillators' motion into oscillations of the magnetic field. This drives spin-flip transitions of trapped atoms to untrapped motional states. The coupling strength is not limited by the square root of the mass ratio of atoms and oscillator as in the other coupling schemes discussed in this thesis. We investigate this coupling scheme theoretically, and discuss the realization of a nanometer-sized mechanical oscillator with a magnetic island. I report on the status of the fabrication, and propose a simplified fabrication method.
This work investigates different coupling mechanisms between ultracold atoms and mechanical oscillators. In a first experiment, atom-surface force is used to couple the vibrations of a mechanical cantilever to the motion of a Bose-Einstein condensate in a magnetic microtrap on a chip. The atoms are trapped at sub-micrometer distance from the cantilever surface. Coupling is used to read out the cantilever vibrations with the atoms. Coupling via surface forces could be employed to couple atoms to molecular-scale oscillators such as carbon nanotubes. In a second experiment, coupling via a 1D optical lattice that is formed by a laser beam retroreflected from the cantilever tip. The optical lattice serves as a transfer rod which couples vibrations of the cantilever to the atoms and vice versa.
We propose and analyze a setup to achieve strong coupling between a single trapped atom and a mechanical oscillator. The interaction between the motion of the atom and the mechanical oscillator is mediated by a quantized light field in a laser driven high-finesse cavity. In particular, we show that high fidelity transfer of quantum states between the atom and the mechanical oscillator is in reach for existing or near future experimental parameters. Our setup provides the basic toolbox from atomic physics for coherent manipulation, preparation, and measurement of micromechanical and nanomechanical oscillators.
In this thesis, I report on the realization of a hybrid optomechanical system in which ultracold atoms are coupled to a micromechanical membrane. The atoms are trapped in the intensity maxima of an optical standing wave formed by retroreflection of a laser beam from the membrane surface. Vibrations of the membrane displace the standing wave, thus coupling to the center-of-mass motion of the atomic ensemble. Conversely, atoms imprint their motion onto the laser light, thereby modulating the radiation pressure force on the membrane. In this way, the laser light mediates a long-distance, coherent coupling between the two systems. \n \nWhen the trap frequency of the atoms is matched to the membrane frequency, we observe resonant energy transfer. In addition, by applying simultaneous laser cooling to the atoms, we can dissipate energy from the coupled system leading to sympathetic cooling of the membrane mode. The experimental data follows the theoretical estimations that predict the coupling to scale with the number of trapped atoms. Furthermore, by including the finite temperature of the atoms and their spatially inhomogeneous trapping potential in the theoretical model of the optomechanical coupling, we can accurately describe the width and shape of the resonance. \n \nIn an improved experimental setup, the membrane is enclosed in a cavity while the atoms are trapped in the standing wave lattice outside the cavity. The presence of the cavity results in a considerable enhancement of the coupling strength in proportion to the cavity finesse. So far we have observed sympathetic cooling of the membrane mode by a factor of 32 starting from room temperature. Theoretical estimates show that in such a setup ground-state cooling of the membrane mode should be possible, allowing one to access the quantum coherent coupling regime.
Techniques to controllably interface atoms with quantum optical fields form the basis for many applications in quantum information science. For example, photons are convenient to relay information over large quantum networks, while atoms naturally are physical systems that can process and store this information. Thus far, the available techniques to efficiently couple single photons with atomic media fall into one of the following, mostly independent, categories: i) cavity quantum electrodynamics (QED), where atomic interactions with light are enhanced via a high-finesse cavity, ii) coherent coupling with atomic ensembles exhibiting large optical depths, and iii) the use of fields tightly focused to dimensions smaller than or approaching the scattering cross-section of a single atom. Although remarkable achievements have been made with all of these approaches, a robust, scalable technique remains elusive. Recently, several groups have successfully demonstrated that cold atoms can be trapped near and interfaced with nanophotonic systems, such as hollow-core photonic crystal fibers [1] and tapered nanofibers [2–4]. The traps are well-characterized [2, 3, 5], and the nearly diffraction-limited transverse confinement of optical fields enables ∼ 10% coupling efficiency of a single atom to the fiber [2, 3]. This has already led to remarkable observations of strong light-matter interactions using relatively few atoms and low powers [1]. It has also been proposed that such techniques can be extended to highly configurable photonic crystal waveguides [6]. Together, these efforts raise the intriguing possibility for future nanophotonic systems with tremendous figures of merit, wherein atom-light interactions can be tailored nearly at will. Here, we discuss recent efforts to develop novel techniques to realize strong, controllable atom-photon and photon-photon interactions, which take full advantage of the parameter space afforded by nanophotonic interfaces and are not based upon the extension of existing techniques. As a specific example, we discuss a protocol to achieve “all-atomic” cavity QED [7], as briefly described below. First, we show that although the single-atom coupling to guided modes of a nanophotonic waveguide might be relatively weak, there exist collective modes of a trapped atomic ensemble whose coupling to light is enhanced by the square root of the atom number, √ NA. While collective effects are generally well-known, special consequences emerge in the nanofiber system when the atoms are trapped in a lattice. In particular, collective effects cause such a lattice to act as a near-perfect mirror for an incident field close to resonance. In analogy to cavity QED, we then demonstrate that two sets of atomic mirrors can form an effective cavity, which can greatly enhance the coupling of a single, specially chosen “impurity” atom (or a few impurity atoms) positioned inside. We introduce a novel quantum spin model to describe the atom-light coupling, which allows one to exactly map the atom-nanofiber interface onto the simple and elegant Jaynes-Cummings model of cavity QED [8]. A unique feature of our atomic mirrors compared to conventional cavities is that they have long relaxation times and are highly dispersive. Remarkably, even with very low mirror finesse (F ∼ 10 2 ), this property allows one to attain the “strong coupling” regime of cavity QED, where vacuum Rabi oscillations [9–12] occur between an excited impurity atom and a single “photon” stored in the cavity (or more precisely, in the atomic mirrors). Furthermore, as quantum mechanical objects, these atom mirrors can be used to store quantum information and transfer this information into propagating waveguide modes. These various features can be combined to realize all of the building blocks for scalable quantum information processing.
In many experiments isolated atoms and ions have been inserted into high-finesse optical resonators for the study of fundamental quantum optics and quantum information. Here, we introduce another application of such a system, as the realization of cavity optomechanics where the collective motion of an atomic ensemble serves the role of a moveable optical element in an optical resonator. Compared with other optomechanical systems, such as those incorporating nanofabricated cantilevers or the large cavity mirrors of gravitational observatories, our cold-atom realization offers direct access to the quantum regime. We describe experimental investigations of optomechanical effects, such as the bistability of collective atomic motion and the first quantification of measurement backaction for a macroscopic object, and discuss future directions for this nascent field.