The goal of this Assessment Project is the study of a number of basic problems the solution of which might pave the way towards an all-optical spin-based quantum gate. Quantum dots offer a variety of spin/charge two-level-systems, which are considered as promising quantum bit candidates for quantum information processing in a solid-state environment. The long-term vision of this proposal is to combine the advantages of spins (long decoherence times) and of charges (short manipulation times by ultra fast laser pulses). In this proposal of a gate, information will be encoded in spins, the spins will be swapped into charges and the charge excitations will be controlled optically. The spin-charge swapping becomes possible by creating a charged exciton as an intermediate state during qubit manipulation. The formation of these complexes is determined by the Pauli exclusion principle. Controllable entanglement of two spin qubits in adjacent quantum dots will be obtained through the Coulomb interaction of the excitons in the dot structures during gating.
Lately a large number of proposals has been made for semiconductor based implementations of quantum information processing, which have attracted considerable interest due to their potential scalability. Many of the proposals rely on spin or charges states in quantum dots. The properties of these excitations have been studied including the time scales on which they decohere. From these studies it has become clear that both spin and charge suffer from severe deficits (slow manipulation and fast dephasing, respectively). Therefore it seems natural to seek for implementations in which the best of both excitation types is combined: In our vision quantum bits would store the information. The swapping of spin to charge will allow for an ultra fast processing. Such a swapping/processing might be achieved by creating the intermediate state of a charged exciton during gating. The objective of this Assessment project is the study of a couple of very basic problems for which answers have to be found in order to move further ahead towards an all-optical spin-based quantum processor.
DESCRIPTION OF WORK
The research work that will be performed during the one year project duration can be summarized by the following four points:
1) Identification of the origin of the damping of the Rabi-oscillations: Here we will obtain a positive answer if we can prove that the damping of the Rabi-oscillations observed for arrays is indeed related to the dot inhomogeneities and does not represent a fundamental limit that is present already on a single quantum dot level. This question can be addressed by studying quantum dot arrays with a smaller inhomogeneous broadening;
2) Identification of a single spin in a quantum dot: Here we aim at identifying the spin of an electron by creating optically the intermediate state of a charged exciton and monitoring the radiative decay of this complex. The possibility to create a negatively charged exciton depends on the spin of the excess carrier in the dot. Due to Pauli-blocking, creation is only possible if the spins of the two electrons are opposite giving rise to optical polarization selection rules. This work also is closely related to swapping spin into charge;
3) Study of the interaction of carriers in quantum dot molecules: Since the entanglement of the spins relies on Coulomb interactions, we aim at demonstrating a control of charges in quantum dot molecules. First, the possibility for fabricating quantum dot molecules of high quality needs to be demonstrated. Second, a controlled entanglement requires the possibility to tailor the spatial charge distribution in the quantum dots which might be obtained by applying electromagnetic fields to the samples;
4) On the theory side there are various issues to be studied: these include quantitative calculations for the laser controlled coherent dynamics of quantum dot systems, and the careful study of decoherence mechanisms (due to phonon decoherence, spontaneous emission). A central part is the calculation of gate fidelities for single and two qubit gates, which allow the evaluation of the fundamental limits of these devices. A guide line in this developments are the theoretical ideas and tools of quantum optics and atomic physics, which - up to a certain point - will provide analogies between 'atoms' and quantum dots as 'artificial atoms and molecules'.
Funding SchemeACM - Preparatory, accompanying and support measures