1. Center for the Physics of Information,
2. Thomas J. Watson Sr Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA
3. Present address: Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.

Correspondence to: Oskar Painter² Correspondence and requests for materials should be addressed to O.P. (Email: opainter@caltech.edu).

Abstract

Cavity quantum electrodynamics¹, the study of coherent quantum interactions between the electromagnetic field and matter inside a resonator, has received attention as both a test bed for ideas in quantum mechanics and a building block for applications in the field of quantum information processing². The canonical experimental system studied in the optical domain is a single alkali atom coupled to a high-finesse Fabry–Perot cavity. Progress made in this system^{1, 2, 3, 4, 5} has recently been complemented by research involving trapped ions⁶, chip-based microtoroid cavities⁷, integrated microcavity-atom-chips⁸, nanocrystalline quantum dots coupled to microsphere cavities⁹, and semiconductor quantum dots embedded in micropillars, photonic crystals and microdisks^{10, 11, 12}. The last system has been of particular interest owing to its relative simplicity and scalability. Here we use a fibre taper waveguide to perform direct optical spectroscopy of a system consisting of a quantum dot embedded in a microdisk. In contrast to earlier work with semiconductor systems, which has focused on photoluminescence measurements^{10, 11, 12, 13, 14}, we excite the system through the photonic (light) channel rather than the excitonic (matter) channel. Strong coupling, the regime of coherent quantum interactions, is demonstrated through observation of vacuum Rabi splitting in the transmitted and reflected signals from the cavity. The fibre coupling method also allows us to examine the system's steady-state nonlinear properties, where we see a saturation of the cavity–quantum dot response for less than one intracavity photon. The excitation of the cavity–quantum dot system through a fibre optic waveguide is central to applications such as high-efficiency single photon sources^{15, 16}, and to more fundamental studies of the quantum character of the system¹⁷.

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