Multidimensional non-linear spectroscopy and coherent manipulation of single and coupled quantum dots

The proposed project spans from the field of optical spectroscopy of semiconductor nanostructures, specifically coherent spectroscopy of single quantum dots, to quantum computing, specifically the implementation of quantum operations in quantum dots. Technological advances in light detectors and microscopy techniques during the last decade have allowed the investigation of the emission properties of individual localized light emitters such as dye molecules, defects in semiconductors, or semiconductor quantum dots. The observation of coherence in these systems and their manipulation by coherent control is presently at the forefront of the research in the field, driven by the expectation that these techniques allow the implementation of quantum information processing using optical transitions in single quantum dots as qubits or to control spin q-bits.
The goal of the project is the application of the recently developed experimental technique of heterodyne spectral interferometry (HSI) [Optics Letters 31, 1151 (2006)] to determine the coherent coupling structure in few-quantum-dot systems by two-dimensional four-wave mixing [Nature Photonics 5, 57 (2011)], and then use this knowledge to design optical control pulses to implement simple quantum gates in the few-quantum-dot system, again using HSI to read the result of the gate operations [Phys. Rev. Lett. 95, 266401 (2005)]. HSI has the advantages of a multi-channel detection of all spectral components simultaneously, a shot-noise limited sensitivity, and a retrieval of amplitude and phase of the FWM signal. These properties are achieved even in the presence of a strong background from the optical excitation pulses, which previously has been avoided by using non-resonant excitation or non-resonant detection, as shown by us in the publications Nature Commun. 4, 1747 (2013), New J. Physics 15, 055006 (2013), New J. Physics 15, 045013 (2013). Having this technique at hand, the resonant coherent control and the implementation of all-optical quantum gates is feasible. The detailed information about the quantum system after the control pulse gained by this detection scheme opens the way to a well defined design of the control pulse according to the system properties, and thus to a quantum gate of high fidelity. This project is supervised by Professor Wolfgang Langbein.