Quantum Optics Seminar by Ralph N. Malein

Spectroscopic investigation of nuclear field effects on single spins in InGaAs quantum dots

Epitaxially-grown semiconductor quantum dots (QDs) hold great promise for quantum information technologies, as both sources of highly coherent single indistinguishable photons, and, via electron and hole spins, as resources for entanglement and quantum memories.  One of the major hurdles to overcome endemic to InGaAs QDs is the hyperfine interaction of single electron or hole spins in the QD with the ensemble of ~10^5 non-zero nuclear spins. These spins are represented by a fluctuating effective magnetic "Overhauser" field, which can cause dephasing of spins in the QD.  We investigate the effect of the Overhauser field on single electron and hole spins both spectroscopically and through mathematical modelling of the 4-level charged exciton states. I will show that the in-plane component of the Overhauser field allows spin-flip transitions between electron spin states at low excitation power, resulting in a broadened doublet in the spectrum, and our modelling shows good agreement with experimental data.  A modest external magnetic field in the Faraday geometry screens the electron spin, recovering expected two-level behaviour.  The in-plane Overhauser field also allows spin-flip transitions between holes, but valence band splitting in InGaAs QDs complicates matters somewhat.  I will describe the expected behaviour of the positively-charged exciton emission at low excitation powers, and show preliminary spectra that support our model.