Photons emitted by quantum dots (QDs) are potentially important in a number of quantum information processing (QIP) applications, but because the extent of their coherence is currently significantly less than ideal, that potential remains severely limited. This is primarily because many proposed QIP applications for photons – e.g., linear optical quantum computation [Knill et al., 2001], entanglement swapping [Zeilinger et al., 1997], and quantum repeaters [Briegel et al., 1998] – are based on the phenomenon of two-photon interference, which we and others have shown depends sensitively on the coherence of the photons involved [Flagg et al., 2010]. The level of coherence of QD photons is known to be reduced by interactions between a QD and its local environment, specifically fluctuations in the nearby charge distribution that cause shifts in the energy of the QD states [Robinson and Goldberg, 2000]. This process is called spectral diffusion. In some cases the level of coherence can be enhanced by coupling the QD to an optical cavity [Santori et al., 2002], but that approach is not suitable for all applications. Consequently, the direct reduction of spectral diffusion at the QD itself would ultimately be necessary if efforts to increase the level of coherence of the emitted photons in all instances are to succeed. Thus, there is a critical need to identify and be able to control the environmental and external sources of spectral diffusion in semiconductor quantum dots. In the absence of such knowledge, the level of coherence and suitability for QIP of any QD-based photon source – be it entangled, on-demand, or otherwise – will likely remain limited.
The objective of this research is to identify, model, and establish effective strategies to address the primary environmental factors responsible for spectral diffusion in QDs, in order to significantly improve their photons’ level of coherence. The central hypothesis is that decoherence is primarily caused by spectral diffusion, which in turn is primarily caused by fluctuating impurity charge-states in the local environment of the QD, and that it can be significantly reduced by a combination of extrinsic doping (to counteract intrinsic doping), the application of electric fields from p-i-n structures, and leveraging resonant excitation of the QD itself. The rationale for this approach is that reducing or eliminating spectral diffusion in a variety of situations is expected to enable the design of QD-based photon sources with significantly higher fidelity performance in a variety of interference-based QIP protocols than is currently possible.
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