QI PhD Defence Notice - Mohd Zeeshan

Wednesday, September 23, 2020 2:00 pm - 2:00 pm EDT

Candidate: Mohd Zeeshan

Title: Quadrupole Electric Field for Erasing the Fine Structure Splitting of a Nanowire Quantum Dot Entangled Photon Source

Date: September 23, 2020

Time: 2:00 PM


Supervisor(s): Reimer, Michael



Semiconductor quantum dots (QD) can generate on-demand entangled photon pairs via biexciton-exciton cascade. These solid-state sources are currently limited in their applications due to the two major challenges,  first is the collection efficiency, since only a fraction of a percent of the photons that are emitted from the quantum dots can be collected due to its isotropic emission profile, and the second is the fine structure splitting (FSS) that causes degeneracy in the intermediate exciton state; it arises due to the strain and structural asymmetry of the quantum dot and degrades its measured entanglement fidelity. Quantum dots have been integrated into photonic structures such as microwave cavities and nanowires to enhance their collection efficiency by coupling the emitted photons to the cavity or wave-guide mode. However, fine structure splitting still remains the major challenge, and although many post-growth perturbation techniques have been implemented to tune and completely erase the FSS of self-assembled QDs, erasing the FSS of a QD in photonic structures has not been achieved yet. In this thesis, we propose and then demonstrate the tuning of the FSS for an InAsP QD embedded inside the tapered InP nanowire source by applying a quadrupole electric field.


In this thesis, we will first propose device design to implement the approach of an electrostatic tuning of the QD FSS by a quadrupole electric field. The novelty of our approach is then emphasized through the numerical simulations, where we show that quadrupole electric field corrects for the spatial asymmetry of the excitonic wave function for any quantum dot dipole orientation (θ= 100, 200) and reduces fine structure splitting (from 11 μeV to 0.05 μeV) without compromising the spatial overlap between electrons and holes (β = 90%). This approach of an external perturbation on the photonic structure is a major step toward developing a deterministic source of entangled photons with high fidelity and collection efficiency.


The second part of this thesis presents the detailed fabrication recipe for the electrical device based on our theoretical proposal. We have developed two different fabrication recipes for the metal gates around the vertical standing InAsP/InP QD/nanowire source, first by UV lithography and second by e-beam lithography.


Finally, we have characterized the devices to study the effect of fabrication on the spectral property of the InAs quantum dot. The electrical characterization of the device shows the FSS tuning from 7 μeV to 4 μeV when we apply a quadrupole electric field to the device fabricated by \gls{uv} lithography.