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Quantum Electronics and Laser Science Conference

Quantum-bit rotations in single quantum dots: Rabi oscillations of excitons and biexcitons

Xiaoqin Li, Todd H. Stievater, Yanwen Wu, Duncan G. Steel

Published Jun 25, 2003• 0 citations
Publisher

Abstract

Summary form only given. We use transient nonlinear optical spectroscopy to demonstrate Rabi oscillations between each of the dipole coupled states, corresponding to qubit rotations. The interface fluctuation GaAs quantum dots are probed with sub-micron spatial resolution. The resonant excitation of the biexciton requires a two-color scheme in a hyper Raman gain configuration. To demonstrate exciton Rabi oscillations, the laser wavelength is fixed at the exciton resonance. Exciton and biexciton Rabi oscillations correspond to qubit rotations of the |00> to |01> or |10> state and the |01> or |10> to |11> state respectively, depending on the polarization of the excitation field.

DOI
10.1109/qels.2002.1031269

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Summary form only given. We use transient nonlinear optical spectroscopy to demonstrate Rabi oscillations between each of the dipole coupled states, corresponding to qubit rotations. The interface fluctuation GaAs quantum dots are probed with sub-micron spatial resolution. The resonant excitation of the biexciton requires a two-color scheme in a hyper Raman gain configuration. To demonstrate exciton Rabi oscillations, the laser wavelength is fixed at the exciton resonance. Exciton and biexciton Rabi oscillations correspond to qubit rotations of the |00> to |01> or |10> state and the |01> or |10> to |11> state respectively, depending on the polarization of the excitation field.
The focus of the thesis is nonlinear transient optical properties and coherent control of single interface fluctuation quantum dots (QD's). When single exciton and biexciton resonances were driven by strong resonant laser pulses, Rabi oscillations of these transitions were observed in the differential transmission (DT) geometry. These results extend the concept of coherent control to the limit of single quantum confined systems in solids and have important implications for the feasibility of building quantum information processing devices using semiconductor QD's. Exciton Rabi oscillations are equivalent to arbitrary single-bit operations. The pulsed control of the exciton-biexciton transition is utilized as the optical operation for building a two-bit controlled rotational gates (CROT) in a single QD. A fidelity of 0.79 was obtained from numerical evaluation of the gate operation itself assuming typical decay parameters measured from these dots. Next, the nonradiative quantum coherence between two orthogonally polarized excitons confined in a single QD was studied. By using pulses with large enough bandwidth and the proper polarization, two exciton states are simultaneously excited. The quantum beats between the two exciton states were temporally resolved as the relative phase between the two dipole oscillations changed. It was found that the nonradiative coherence time is limited by the lifetimes of the excitons involved even in the strong field regime. In the basis of the pseudo-spin Bloch vectors of exciton transitions, the total wavefunction following the pulsed excitation is an entangled state, i.e. the wavefunction is non-factorizable. Finally, the quantum coherence between the biexciton, exciton, and the crystal ground states was studied in a novel quantum beat experiment, where the oscillations only existed when the pump and probe pulses overlapped in time. Laser pulses generated after a passive pulse shaper with just enough bandwidth to cover both the exciton and biexciton transitions were used in this experiment. The appearance of the beats is indicative of a reasonably long coherence time between the biexciton and other states even in the presence of strong pulsed excitations.
Transient nonlinear optical spectroscopy, performed on excitons confined to single GaAs quantum dots, shows oscillations that are analogous to Rabi oscillations in two-level atomic systems. This demonstration corresponds to a one-qubit rotation in a single quantum dot which is important for proposals using quantum dot excitons for quantum computing. The dipole moment inferred from the data is consistent with that directly obtained from linear absorption studies. The measurement extends the artificial atom model of quantum dot excitonic transitions into the strong-field limit, and makes possible full coherent optical control of the quantum state of single excitons using optical pi pulses.
Summary form only given. Recent studies have been carried out on the resonant response from single interface fluctuation quantum dots. Such a discrete-level model yields unique signatures under strong resonant driving, including Rabi oscillations. To date, Rabi oscillations (excluding microcavity studies) have only been reported for two-dimensional excitonic systems. We present data showing the nonlinear response from an ensemble of interface fluctuation quantum dots. This data is consistent with Rabi oscillations for moderate driving fields (that is, for /spl Theta//spl sim/2/spl pi/, where /spl Theta/ is the pulse area). The sample under investigation is a single thin (62 /spl Aring/) GaAs/Al/sub 0.3/Ga/sub 0.7/As quantum well grown with growth interruptions and is studied at 5 K. The laser is tuned to the lower energy excitonic resonance, corresponding to an inhomogeneously broadened distribution of excitons localized due to well width fluctuations. The time-integrated PL is collected with a spectrometer tuned 2.5 meV below the excitonic resonance.
Summary form only given. A single quantum dot (QD) exciton is considered as a basic unit capable of carrying one bit of quantum information (qubit). Reference has demonstrated exciton Rabi oscillations in single QD's that are equivalent to single-bit rotations. To realize two-bit quantum logic operations in the next step, it is essential to identify and coherently manipulate two interacting single QD excitons. Our earlier work shows that two-exciton entangled states not involving the biexciton can be optically induced. This paper extends the entanglement to include the biexciton and show that all four basis states of the two-exciton system can be coherently manipulated based on a coherent nonlinear optical spectroscopy technique.
Measurements of optical Rabi oscillations in the excited states of individual InGaAs are presented. Under pulsed resonant excitation we observe Rabi oscillations with increasing pulse area, which are damped after the first maximum and minimum. We show that the observed damping comes from an additional non-resonant generation of carriers in the quantum dot. The observation of Rabi oscillations provides an efficient way of directly measuring the excitonic transitions' dipole moments. A polarization anisotropy of the dipole moment is resolved in some of the quantum dots.