Increasingly complex structures such as optical antennas or cavities are coupled to self-assembled quantum dots to harvest their quantum-optical properties. In many cases, these structures pose a problem for common methods of ultrafast spectroscopy used to write and read out the state of the quantum dot. We present a pure far-field method that only requires optical access to the quantum dot and does not impose further restrictions on sample design. We demonstrate Rabi oscillations and perturbed free induction decay of single GaAs quantum dots that have a dipole moment as small as 18 D. Our method will greatly facilitate ultrafast spectroscopy of complex quantum-optical circuits.

The controlled interaction of several quantum dots (QDs) mediated by plasmonic or photonic nanostructures promises interesting new functionality in the fields of quantum computing and telecommunication. Ultrafast laser pulses can be used to write and read out the state of the QD. We review ultrafast coherent spectroscopy of single QDs. The focus of this article is on the technique of transient reflection spectroscopy which can be applied to a broad range of samples and devices. It only requires optical access to a single quantum system next to a reflecting surface. We demonstrate the versatility of our approach by presenting several quantum optical studies such as Rabi oscillations, perturbed free induction decay, and quantum beats from an entangled excitonic state in weakly absorbing GaAs QDs. We expect this experimental method to make coherent experiments possible in elaborate devices where quantum emitters are interacting with a complex environment such as plasmonic waveguides or antennas. 1 Introduction In recent years substantial efforts were made to produce matter quantum bits (standing qubits) in various systems [1]. These quantum mechanical two-level systems can be realized by atom-like systems such as single molecules, nitrogen vacancy centers in diamond or semiconductor quantum dots (QDs). Photons are acting as flying quantum bits (qubits) which can be used to transfer quantum information from one stationary qubit to the next and in this way form the connections in a quantum network. Artificial atoms are coupled to cavities or waveguides in order to establish an interface between photons and excitations in matter. In order to produce new photonic devices with novel functionalities one needs not only an efficient quantum system that can be addressed by photons but also controlled coupling to the interconnects.Semiconductor QDs are appealing systems in this context as they are a pure solid-state system which makes them stable, long-lived, and compatible with conventional 2D fabrication techniques. Dephasing times of neutral excitons in QDs can be on the order of several hundreds of

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