Abstract
Several important proposals to use semiconductor quantum dots in quantum information technology rely on the control of the dark exciton ground states, such as dark exciton based qubits with a $\mu$s life time. In this paper, we present an efficient way to occupy the dark exciton ground state by a single short laser pulse. The scheme is based on an optical excitation with a longitudinal field component featured by, e.g., radially polarized beams or certain Laguerre-Gauss or Bessel beams. Utilizing this component, we show within a configuration interaction approach that high-energy exciton states composed of light-hole excitons and higher dark heavy-hole excitons can be addressed. When the higher exciton relaxes, a dark exciton in its ground state is created.
Highlights
With their discrete energy states, semiconductor quantum dots (QDs) are destined to play an important role in solidstate quantum information technology [1], for example, as sources for single or entangled photons [2,3,4,5,6] or as realizations of qubits [7,8,9]
Our scheme utilizes light beams with a pronounced longitudinal component tuned in resonance to certain higher exciton states
These states are characterized by a strong valence-band mixing between optically active excitons with LH ± 0 spin configurations and higher dark excitons with HH ± 2 spin configurations, which enables a large coupling to the light field and, at the same time, an ultrafast relaxation path into the dark exciton ground state
Summary
With their discrete energy states, semiconductor quantum dots (QDs) are destined to play an important role in solidstate quantum information technology [1], for example, as sources for single or entangled photons [2,3,4,5,6] or as realizations of qubits [7,8,9]. Weak mixing between bright and dark excitons can be induced either by applying an external magnetic field [18,19] or by breaking the C2v symmetry of the QD [20] This makes a direct optical excitation of the dark exciton possible [21], with an oscillator strength which is some orders of magnitude smaller than the absorption into the bright exciton. Valence-band mixing couples those LH excitons preferentially to higher heavy-hole (HH) excitons involving a dark spin configuration, building strongly mixed higher exciton states Due to their dark HH exciton contribution, these states will relax by phonon emission preferentially into the dark HH exciton ground state. Considering higher excited HH-shells, such as the d shell, we will show that these couplings become strong and enable the proposed efficient scheme to excite the dark HH ground state.
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