Techniques to Improve Quantum Dot Emission: Resonant Two-Photon Excitation and Optical Frequency Shifting via Electro-optic Modulation

Candidate: Turner Garrow
Title: Techniques to improve quantum dot emission: Resonant two-photon excitation and optical frequency shifting via electro-optic modulation
Date: October 12, 2021
Time: 11:00 am
Place: MS Teams
Supervisor(s): Reimer, Michael

Abstract:

The ability to create pairs of entangled photons is a requirement for many near-future quantum technologies. Despite this, the current state-of-the-art entangled photon sources are fundamentally limited in their performance by their probabilistic nature. Recently, semiconductor quantum dots have gained a great deal of interest as candidates for next-generation entangled photon sources since quantum dots produce photon pairs deterministically, and are therefore not fundamentally limited in their performance. The results presented here are from two separate experiments relating to the emission properties of a quantum dot. The first is resonant two-photon excitation of the quantum dot, a scheme of optically exciting the dot which is expected to outperform all other optical excitation methods. By directly exciting charges within the quantum dot through two-photon excitation, the charge noise is decreased, which reduces both re-excitation of the dot and dephasing over the lifetime of the excited state. Quantum state tomography of the emitted pairs reveals a peak concurrence of 0.87(4), and represents the first ever tomography measurement of a nanowire quantum dot excited with this excitation scheme.
 

The second experiment presented here is related to an all-optical method of eliminating the quantum dot fine structure splitting. The fine structure splitting is an energy difference between the intermediate states of the optical cascade, and is an unwanted property of semiconductor quantum dots introduced in the fabrication process. The proposed method uses a pair of electro-optic modulators to shift the energy of the emitted photons to compensate for this energy difference. Here, we present results from a lithium niobate electro-optic modulator, capable of frequency conversion of circularly polarized light with an average efficiency of 82.2%. This demonstration shows that an all-optical fine structure eraser is feasible, and leaves us well-positioned for an experimental demonstration in the near future.