Ph.D. Research Proposal Exam: Harjot Singh
Monday, October 18, 2021
ERP 1207 (Ireap Large conference room)
301 405 3681
ANNOUNCEMENT: Ph.D. Research Proposal Exam
Name: Harjot Singh
Professor Edo Waks (Chair)
Professor Mohammad Hafezi
Professor Cheng Gong
Date/time: Monday, October 18, 2021 at 4pm-5pm
Location: ERP 1207 (Ireap Large conference room)
Title: Strong optical nonlinearity and spin control using photonic structures
Abstract: Photons don’t interact with each other inherently but indirect interactions between them can be generated by the use of optical non-linearities. To generate interactions between photons at the single photon level, we need to use optical nonlinearities at the single photon level. Optical nonlinearity at the single photon level can be created by engineering a strong interaction between a two level atom and one mode of the electromagnetic radiation. A single mode optical cavity coupled to a two level atom enables this strong interaction. However, photon-photon interactions induced by an atom-cavity system are fundamentally limited due to time energy uncertainty. One manifestation of this fundamental limit occurs in the efficiency of photon-photon splitting. I will show how the limit in the efficiency of photon-photon splitting can be overcome by performing a linear optical transformation on the modes of the two-level atom. I will show the photonic structures I have developed to demonstrate this limit and its overcoming using InAs/GaAs quantum dots as two-level atoms.
It turns out that if the atom which is coupled to an optical cavity has metastable ground states and realizes a lambda system, it can create deterministic interactions between two photons. An InAs/GaAs quantum dot charged with an electron realizes such a lambda system where the two spin states of the electron realize the desired metastable ground states. Electron spin in a charged InAs/GaAs interacts with the surrounding nuclear spins of Ga, In and As through a non-collinear hyperfine interaction, limiting the coherence time of the spin states. This also places a limit on the fidelity of the photon-photon interactions which can be generated using this system. Therefore, it is very important to identify the effect of the unwanted interactions, which effectively act as a non-markovian noise for the electron spin and to decouple the dynamics of the electron spin from this noise. I will present the work we have done so far in measuring these noise terms and decoupling the electron spin from them. I will show the existence of a laser induced relaxation of the electron spin which limits the number of operations we can perform. Application of a high power laser to perform operations on the electron spin probably causes changes in the charge environment of the quantum dot, which leads to the relaxation of the electron spin. Since the electron spin is driven with circularly polarized radiation, we may reduce the effect of laser induced relaxation by coupling the quantum dot to photonic structures which support circular polarization. Bullseye antennas are one example of photonic structures which can support circular polarization. I will present the design of bullseye structures, fabrication of these structures and what we’ve learned so far by measuring their response to light.