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March 2006 EEEL Measures Homogeneous Linewidth of Quantum Dots |
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Joe Berry and colleagues Kevin Silverman, Martin Stevens, and Rich Mirin of the Optoelectronics Division of EEEL have used a novel, pump-probe, spectral-hole-burning technique to perform high-resolution spectroscopy of the homogeneous linewidth of self-assembled InGaAs/GaAs quantum dots. The measurements were made by coupling light from two narrow-frequency diode lasers into a waveguide containing a single layer of quantum dots. The probe laser beam was swept in frequency relative to the pump, and signal discrimination of the differential absorption was achieved using a heterodyne technique. The researchers measured the pump power-dependent broadening of the homogeneous linewidth and obtained a full-width-at-half-maximum at a low pump power of 0.74 eV, corresponding to a coherence time T2 of 1.8 ns. Measurement techniques used by others have not circumvented dephasing mechanisms induced by the inhomogeneous distribution of dipole moments of the quantum dots or are affected by other factors such as static electric fields or doping of the dots. This effort forms part of the work within the quantum information community to find the most suitable material to be used as a qubit. Knowledge of the coherence time is essential, but very challenging, in part because the characteristics of the quantum dots vary during the time intervals needed to perform the experiments (typically seconds or longer). The technique used by Berry et al. is largely immune to any time-dependent variations in the quantum dots. The measured coherence time is about twice as long as that measured in previous studies of InGaAs quantum dots by others and yields a high figure of merit for quantum computing applications. It potentially enables a large number of quantum operations before the optically excited charges decay (~ 1 ns). Reference: Berry, J. J., Stevens, M. J., Mirin, R. P. & Silverman, K. L. "High-resolution spectral hole burning in InGaAs-GaAs quantum dots," Appl. Phys. Lett. 88, 061114 (2006).
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