PhD defence - Milad Khoshnegar Shahrestani

Tuesday, January 14, 2014 1:00 pm - 1:00 pm EST (GMT -05:00)


Milad Khoshnegar Shahrestani


Nanowire Quantum Dots as Sources of Single and Entangled Photons


Majedi, Amir H.


Developing solid-state nanostructures able to deterministically generate the quantum state of light has been an elemental concern since the emergence of quantum information science. Realization of linear quantum computation and establishing secure quantum communication among interacting parties demand for triggered quantum sources delivering genuine single and entangled photons. However, the intrinsic energy level spectrum of nanostructures made by the nature or developed under a random growth process energetically lacks the expected figures of merit to produce such quantized states of photons.

Here, I present the semi-empirical modeling and experimental investigation on the spin fine structure of strongly confining quantum dots embedded in III-V nanowires. To this end, the quantum dot is numerically modeled via the Configuration Interaction method at two different levels: 1) single-particle level, where its pure energy level structure is resolved in the presence of strain and spin-orbit interaction. 2) Few-particle level, at which the few-body interactions appear as perturbative energy corrections and orbital correlations. I demonstrate the influence of quantum confinement on the binding energies and spin fine structure of excitons in the absence of hyperfine interaction. Importantly, the high-symmetry character of excitonic orbitals in nanowire quantum dots restore the degeneracy of optically-active ground-state excitons, offering an ideal spectrum for the entangled photon pair generation.

To experimentally verify the idea, we design and fabricate defect-free nanowire quantum dots with ultra-clean excitonic spectrum, and construct the time correlation function of emitted photons through performing a series of low-temperature statistical quantum optics measurements. We observe a decent performance in terms of single photon generation under low excitation powers. Moreover, photon pairs emitted from the biexciton-exciton cascade of nanowire quantum dots exhibit color indistinguishability and polarization entanglement owing to the trivial fine structure splitting of the ground-state excitons.

We further extend the idea by proposing the hybridized states of a nanowire-based quantum dot molecule as the potential source of higher-order entangled states. Tracing the field-dependent spectrum suggests the appearance of dominant features under the weak localization of electrons and coherent tunneling of holes. In addition to their Coulomb correlation, excitons also remain spatially correlated, opening new transition channels normally forbidden in the ground state of a typical single dot. We particularly show how sensitively the size and interdot spacing tailor the orbital hybridizations and the energy coincidence of absorption energies. The proposed structure can be exploited to create tripartite hybrid, GHZ and W-entangled states.