Linear Optics Quantum Information Technology

Project description

The main objective of this project is to establish an in-depth understanding of the physical mechanisms governing the nanowire single-photon source (SPS). Important applications for a high-efficiency SPS are linear optics quantum computing (LOQC) [1] based on the interaction of single photons, and quantum cryptography [2], where the quantum mechanical properties of single photons are exploited to obtain 100 % secure communication. Both applications put strict demands on the SPS efficiency, defined as the fraction of photons collected per trigger. In LOQC an efficiency of ~ 99 % is a requirement [1] and in quantum cryptography the efficiency directly governs the transmission length over which secure communication is possible [3].

A promising candidate for a high-efficiency SPS is the light-emitting diode with a quantum dot (QD) embedded in a semiconductor microstructure [4]. With careful design the multi-photon emission probability of a QD can be reduced to ~ 0, however to obtain a high efficiency the physical mechanisms governing the coupling of the QD to the optical field and the subsequent escape of the photon must be carefully analyzed. Two parameters β and γ describe these processes [5]. β is the fraction of light coupled to the optical mode interest, γ is the power measured by the detection optics over that of the optical mode, and the total efficiency is given by the product βγ

Fig. 1. Illustration (left) and scanning electron micrograph (right) of the nanowire SPS. 

In this project a new SPS device based on a QD embedded in a semiconductor nanowire [6] will be theoretically explored. The device is shown in Fig. 1. The main idea is to replace the high-Q cavity of the existing micropillar SPS with low-diameter (~ 210 nm) GaAs nanowire where a screening effect is used to deplete radiation modes and ensure a high β [7]. The purpose of this replacement is specifically to establish a high tolerance towards realistic fabrication-induced geometry imperfections such as oxidation, roughness and side wall inclination. Since there is no cavity to confine the photon in the nanowire, it escapes immediately and scattering losses experienced by the photon due to geometry imperfections should be greatly reduced. With this high tolerance towards imperfections, the demands on experimental fabrication are relaxed and the probability of obtaining high efficiency and a good agreement between the theoretically predicted and experimentally observed performance should be greatly improved.

The nanowire SPS consists of a GaAs cylinder with an embedded QD placed on a substrate. Even though the QD couples strongly to the fundamental guided mode of the cylinder, half of the light propagates towards the substrate, so a highly reflecting bottom mirror should be implemented. Standard DBR mirrors are inefficient at low diameters, but high reflectivity coefficients have been reported for a metal-silica interface [8]. Because of the very small cylinder diameter, the forward propagating mode profile has a narrow waist resulting in a highly divergent emission profile. A possible remedy is the implementation of a conical tapering section above the QD which adiabatically extends the mode waist and ensures a high γ [9]. However, to be of interest in applications electrical contacts should be added. A minimal deterioration of γ by the contacts is expected by employing the conducting but weakly scattering Indium-Tin-Oxide (ITO) material. 

Fig. 2. Optical field profile of the nanowire SPS. 

The challenges of this project consist not only of understanding the physics of the various building blocks of the SPS nanowire design but also the interaction of these elements when put together. The element-splitting approach is a good initial approximation, however the β factor is strongly influenced by the optical environment and in particular the presence of a metal mirror and electrical contacts will greatly influence β. It is thus necessary to develop an understanding of how β depends on an advanced photonic environment. This involves an evaluation of the local density of states (LDOS) to determine the spontaneous emission rates into the guided and the radiative optical modes. Also, even though the cavity is absent, multiple-scattering effects due to the combination of the bottom mirror and the top tapering may still occur, and their influence will be studied. The influence of the ITO contacts on the propagation of the optical mode will be analyzed and the far field emission profile will be computed. Finally, the influence of roughness and the improved tolerance towards geometry imperfections is to be quantified.

The project is taking place in close collaboration with the Commisariat à l’Energie Atomique (CEA) in Grenoble, France. The CEA is pursuing the experimental realization of the nanowire SPS and the collaboration thus allows an exchange of design ideas and experimental feedback.

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[9] N. Gregersen, T. R. Nielsen, J. Claudon, J. M. Gérard and J. Mørk, Opt. Lett. 33, 1693-1695 (2008).