| Description |
Single, indistinguishable photons are at the heart of applications in quantum communication, quantum networks and linear quantum computing [1]. Promising light sources that emit such photons are semiconductor quantum dots (QDs). They offer a very good suppression of multiple photon emission, bear the possibility of emitting indistinguishable photons [2], and can be placed into optical resonators for high extraction efficiencies [3, 4].The reduction of the radiative lifetime in these resonators via the Purcell effect is also beneficial for the emission rate and suppression of dephasing [5].
In this talk, I’ll review the design criteria and performance of such sources. Particular emphasis will be given to the excitation scheme, which is especially important for the generation of indistinguishable photons [6].Non-resonant excitation in the semiconductor matrix around the quantum dots leads to a large timing jitter of the emitted photon pulses since the carriers have to relax into the excitonic ground state of the quantum dots. In addition, the charges around the dot induce a quick dephasing of the exciton, reducing the coherence time of the emitted photons. Generation of charged carriers outside the dot can be avoided by exciting higher order states in the quantum dot (p-shell excitation). The best results with respect to the coherence time and indistinguishability have been obtained with strictly resonant excitation [7].
I’ll also discuss ways to deal with a major drawback of self-assembled quantum dots, which is their random position. In order to identify devices with a good alignment between dot and resonator, large numbers of structures have to be screened. I’ll present two approaches that address this issue: the use of site-controlled quantum dots [8] and in-situ lithography of the resonator. For the fabrication of site-controlled quantum dots, nanoholes are patterned on the semiconductor surface. These holes serve as nucleation centers for the subsequent growth of the quantum dots. The dots are now at defined positions relative to alignment markers, and the resonators can be placed to have the dots at the maximum of the optical mode. For the second approach, the sample is coated with optical resist and suitable quantum dots are located during a photoluminescence scan. A laser pulse then exposes the resist, defining the position of the resonator. For devices fabricated with this approach, we achieve nearly-perfect single photon emission with large (>75%) extraction efficiencies and close to 90%indistinguishability in two-photon interference experiments.
Funding is acknowledged by the State of Bavaria and the BMBF within the Q.com-H project.
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