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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Cavity QED with chip-based toroidal microresonators

Barak Dayan, T. Aoki, E. Wilcut, Scott I. Kelber, Warwick P. Bowen, A. S. Parkins, J. R. Petta, Tobias Jan Kippenberg, Eric P. Ostby, Kerry J. Vahala, Harry J. Kimble

Published Sep 11, 2007• 1 citation
Publisher

Abstract

We report the demonstration of strong coupling between single Cesium atoms and a high-Q chip-based microresonator. Our toroidal microresonators are compact, Si chip-based whispering gallery mode resonators that confine light to small volumes with extremely low losses, and are manufactured in large numbers by standard lithographic techniques. Combined with the capability to couple efficiently light to and from these microresonators by a tapered optical fiber, toroidal microresonators offer a promising avenue towards scalable quantum networks. Experimentally, laser cooled Cs atoms are dropped onto a toroidal microresonator while a probe beam is critically coupled to the cavity mode. When an atom interacts with the cavity, it modifies the resonance spectrum of the cavity, leading to rejection of some of the probe light from the cavity, and thus to an increase in the output power. By observing such transit events while systematically detuning the cavity from the atomic resonance, we determine the maximal accessible single-photon Rabi frequency of Ω0/2π ≈ (100 ± 24) MHz. This value puts our system in the regime of strong coupling, being significantly larger than the dissipation rates in our system.

DOI
10.1117/12.734875

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We report the demonstration of strong coupling between single Cesium atoms and a high-Q chip-based microresonator. Our toroidal microresonators are compact, Si chip-based whispering gallery mode resonators that confine light to small volumes with extremely low losses, and are manufactured in large numbers by standard lithographic techniques. Combined with the capability to couple efficiently light to and from these microresonators by a tapered optical fiber, toroidal microresonators offer a promising avenue towards scalable quantum networks. Experimentally, laser cooled Cs atoms are dropped onto a toroidal microresonator while a probe beam is critically coupled to the cavity mode. When an atom interacts with the cavity, it modifies the resonance spectrum of the cavity, leading to rejection of some of the probe light from the cavity, and thus to an increase in the output power. By observing such transit events while systematically detuning the cavity from the atomic resonance, we determine the maximal accessible single-photon Rabi frequency of Ω0/2π ≈ (100 ± 24) MHz. This value puts our system in the regime of strong coupling, being significantly larger than the dissipation rates in our system.
We investigate the suitability of toroidal microcavities for strong-coupling cavity quantum electrodynamics (QED). Numerical modeling of the optical modes demonstrate a significant reduction of the modal volume with respect to the whispering gallery modes of dielectric spheres, while retaining the high-quality factors representative of spherical cavities. The extra degree of freedom of toroid microcavities can be used to achieve improved cavity QED characteristics. Numerical results for atom-cavity coupling strength ggg, critical atom number N0{N}_{0}N0​, and critical photon number n0{n}_{0}n0​ for cesium are calculated and shown to exceed values currently possible using Fabry-Perot cavities. Modeling predicts coupling rates $g∕2\ensuremath{\pi}$ exceeding 700MHz700\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}700MHz and critical atom numbers approaching ${10}^{\ensuremath{-}7}$ in optimized structures. Furthermore, preliminary experimental measurements of toroidal cavities at a wavelength of 852nm852\phantom{\rule{0.3em}{0ex}}\mathrm{nm}852nm indicate that quality factors in excess of 108{10}^{8}108 can be obtained in a $50\text{\penalty1000-\hskip0pt}\ensuremath{\mu}\mathrm{m}$ principal diameter cavity, which would result in strong-coupling values of $\mathbf{(}g∕(2\ensuremath{\pi}),{n}_{0},{N}_{0}\mathbf{)}=(86\phantom{\rule{0.3em}{0ex}}\mathrm{MHz},4.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4},1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3})$.
Of the many applications of whispering-gallery mode (WGM) microresonators, Single-atom cavity-QED poses the most extreme demands on mode-volume and quality factor. Here we present a model-based procedure for the fabrication of small mode-volume ultrahigh Q silica WGM microresonators of varying geometries, from toroidal to micro-spheres. We experimentally demonstrate WGM resonators with ultra-high qualities as high as 1.7×1081.7\times 10^81.7×108 at 780nm. We present a theoretical model that allows tailoring the recipe to attain the desired geometry of the fabricated WGM microresonators.
Of the many applications of whispering-gallery-mode (WGM) micro-resonators, single-atom cavity quantum electrodynamics (cavity-QED) poses the most extreme demands on mode volume, dimensions, and quality factor ( Q ). Here we present the procedure for the fabrication of chip-based, small mode-volume, ultrahigh- Q silica WGM micro-resonators, designed for single-emitter cavity-QED. We demonstrate micro-resonators at varying geometries, from toroidal to micro-spheres, yielding ultrahigh-qualities as high as 1.7 × 10 8 at 780nm. We present a comprehensive theoretical model that allows tailoring the fabrication process to attain the desired micro-resonator geometry.
Cavity quantum electrodynamics (cavity QED) describes electromagnetic fields in a confined space and the radiative properties of atoms in such fields. The simplest example of such system is a single atom interacting with one mode of a high-finesse resonator. Besides observation and exploration of fundamental quantum mechanical effects, this system bears a high potential for applications quantum information science such as, e.g., quantum logic gates, quantum communication and quantum teleportation. In this thesis I present an experiment on the deterministic coupling of a single neutral atom to the mode of a high-finesse optical resonator. In Chapter 1 I describe our basic techniques for trapping and observing single cesium atoms. As a source of single atoms we use a high-gradient magneto-optical trap, which captures the atoms from background gas in a vacuum chamber and cools them down to millikelvin temperatures. The atoms are then transferred without loss into a standing-wave dipole trap, which provides a conservative potential required for experiments on atomic coherence such as quantum information processing and metrology on trapped atoms. Moreover, shifting the standing-wave pattern allows us to deterministically transport the atoms (Chapter 2). In combination with non-destructive fluorescence imaging of individual trapped atoms, this enables us to control their position with submicrometer precision over several millimeters along the dipole trap. The cavity QED system can distinctly display quantum behaviour in the so-called strong coupling regime, i.e., when the coherent atom-cavity coupling rate dominates dissipation in the system. This sets the main requirements on the resonator's properties: small mode volume and high finesse. Chapter 3 is devoted to the manufacturing, assembling, and testing of an ultra-high finesse optical Fabry-Perot resonator, stabilized to the atomic transition. In Chapter 4 I present the transportation of single atoms into the cavity and their coupling to the cavity mode. The strong coupling manifests itself in a strong reduction of the cavity transmission probed by a weak external laser. The atoms remain trapped and coupled to the cavity mode for several seconds until we move them out of the cavity for final analysis of their number and position.
We have used a fiber Fabry-Perot cavity on an atom chip to obtain strong, extremely stable atom-cavity coupling for single atoms and Bose-Einstein condensates trapped inside the cavity. The coherent atom-cavity coupling parameter, g0/2pi ~ 209 MHz, is larger than both cavity decay, kappa/2pi ~ 53 MHz and spontaneous emission gamma/2pi ~ 3 MHz. Due to the small mode volume, these values, well inside the strong coupling regime of cavity QED, are realized with a cavity finesse F ~ 36000, which is a moderate value compared to usual strong-coupling cavity QED experiments. The cavity also achieves a very high cooperativity parameter, C=g02/2kappagamma ~ 130.The use of trapped Bose-Einstein condensates (BECs) is a fundamentally new situation for cavity QED, which should enable a new class of experiments, where atoms that share a well-defined internal and external quantum state collectively interact with a quantized mode of the cavity. Besides, as the spatial extension of the BEC assumes the fundamental limit that can be attained for an atom cloud - much smaller than an optical wavelength in our case - an extremely strong, stable and uniform coupling to the cavity mode is achieved. This makes the system interesting for applications such as quantum memories. For single atoms, our combination of high resonator cooperativity with extreme positioning precision enables qubit detection with near-unit efficiency. This realizes a long-awaited milestone for quantum information processing on atom chips.