Under a degenerate two-photon resonant excitation, the Rabi oscillation of the four-level biexciton system in a semiconductor quantum dot is theoretically investigated. The influence of the laser phases on the state manipulation is modeled and numerically calculated. Due to the interference between different excitation paths, the laser phase plays an important role and can be utilized as an alternate control knob to coherently manipulate the biexciton state. The phase control can be facilely implemented by changing the light polarization via a quarter-wave plate. 1. Introduction The characteristic of atomic-like discrete energy levels, together with the ability of integration in solid-state devices, makes quantum dots (QDs) very promising for solid-state implementations of quantum information processing (QIP) [1, 2]. Therein, the biexciton system, consisting of two mutually coupled excitons in QDs, has attracted much interest as it carries prospects in realization of two-bit quantum logic gates [3, 4] and in generation of entangled photon pairs [5, 6]. Coherent manipulation of quantum states is a basic issue for QIP. Among the state manipulation, Rabi oscillation is a fundamental phenomenon by coherent optical coupling, which provides a direct control to the state population and coherence of a quantum system. Rabi oscillations have been well studied theoretically and experimentally both for exciton and biexciton systems in a QD [3, 4, 7–12]. Rabi oscillations are typically measured by controlling the pulse-area, via the field intensity (excitation power) of the incident laser pulse. Excitation for the biexciton system is in a closed-loop four-level scheme shown in Figure 1, where the ground state is coupled in a V-type structure to intermediate exciton doublet states and , which are themselves coupled to a common excited biexciton state in a - structure. The biexciton state can be excited via by linearly polarized photons or by linearly polarized photons. Previous studies related to biexciton were normally done by selecting one of the two excitation paths with proper polarized light. Herein biexciton Rabi oscillation is simply a function of the field intensity of the light. Now, supposing under the excitation of a light with both and components, then both the two excitation paths will be activated. In a closed-loop configuration, the two excitation paths can interfere, and the state control is dependent on the relative phase between the two paths [13–16]. Figure 1: Scheme of optical transitions for the biexciton system in a quantum dot. See text for
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