Kolodka R. Coherent properties of a single quantum dot exciton embedded in a photodiode

The dissertation is dedicated to the study of the coherent properties of a two-level system based on the ground state neutral exciton of a single InGaAs self-assembled quantum dot (QD) embedded in an n-i-Schottky diode and the possibility of realization of a quantum gate on its basis. An instrument with the capabilities of optical resonant excitation of QDs and measurement of the final quantum state of QDs electrically has been fabricated. Rabi oscillations of the neutral exciton population has been observed (for pulse-areas of up to 6π) and thus tested the possibility of describing a QD as a quantum two-level system. Quantum-confinement Stark effect and quantum interference effect were recorded in photocurrent measurements. Strong nonlinear absorption in the excitation profile of a single InGaAs QD exciton driven by a rectangular spectrum optical pulse, in a region of detuning where the linear response is weak has been observed. Here the exciton is resonant with the temporal side-lobes of the pulse, which drive a damped Rabi oscillation. Good agreement is found between experiment and a two-level atom model. In the nonlinear regime the excitation profile exhibits features from both the rectangular spectrum and the two peaks present in the spectrum of the autocorrelation of the pulse. Using excitation by two consecutive optical pulses the mechanism of exciton absorption recovery in a QD was investigated. In combination with the measurements of quantum interference, this made it possible to divide the contributions of the lifetime and the loss of the phase of the system into the time of coherence. The coherence decay of a single-exciton-based qubit with electrical readout is investigated. Pairs of laser pulses initialize the qubit state and then map the target basis onto the measurement basis, probing the decay of either the population inversion or electronic polarization components of the state vector. A comparison of coherence and population decay confirms that the exciton coherence is lifetime limited by fast voltage-tunable electron tunneling. Optimum coherence times are limited by a heavy-hole tunneling rate slow compared with the repetition rate of the laser.