Proposal for high-speed and high-fidelity electron-spin initialization in a negatively charged quantum dot coupled to a microcavity in a weak external magnetic field

Recent demonstrations of cavity quantum electrodynamics (QED) effects with semiconductor quantum dots (QDs) coupled to microcavities [1–5] show that these systems are robust and scalable platforms for quantum information science. The QD-cavity QED experiments performed so far treat each QD as a two-level quantum system consisting of a ground state and the single exciton excited state. However, multilevel quantum systems are useful for fast, complex, and high-fidelity quantum information processing [6]. Each of these systems should contain two stable ground states between which the transition is dipole forbidden and one or more excited states serve as a passage for population transfer between the ground states ( system) [7,8]. Initialization, coherent manipulation, and readout of these systems are essential for quantum information processing, including controlled phase gate [9,10], quantum repeaters and networks [11], and remote entanglement distribution [12]. Therefore, realizing a multilevel system in a QD coupled to a cavity is crucial for semiconductor cavity QED implementation of quantum computers. One way to realize a system in a negatively charged QD is by employing the Zeeman-split electron-spin states as the ground states and a trion state as the excited state [6]. Recent experiments with QDs (not coupled to a cavity) have demonstrated spin initialization with magnetic field along the QD growth direction (Faraday geometry) [13] as well as with field perpendicular to the QD growth direction (Voigt geometry) [7,14,15]. Initialization in the Faraday geometry requires weak magnetic field and can achieve very high fidelity, but the initialization is slow because the process relies on random spin-flip Raman scattering. On the other hand, initialization in the Voigt geometry is fast, but a strong magnetic field is required to mix the spin states and split the resulting eigenstates for high fidelity. The high cost and large space required to achieve a strong magnetic field present a significant drawback of this initialization method. In this paper, we propose a fast and high-fidelity spin initialization method for a negatively charged QD coupled to a microcavity. Our method also works for a positively charged QD [16], but we will focus on the negatively charged one as a specific example in this paper. The major advantage of our method is the absence of a strong magnetic field because there is no need to mix the electron or the hole spin states.