Advanced micro-photoluminescence spectroscopy of pyramidal site-controlled quantum dots as spin-photon interfaces
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Full Text E-thesis
Date
2025
Authors
Mattana, Francesco
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Publisher
University College Cork
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Abstract
Semiconductor quantum dots (QDs) are low dimensional system, typically a few nanometers in size, characterized by discrete energy levels and strong light - matter interaction.
Due to their optoelectronics properties they have emerged as a promising candidate for qubit system and platform for quantum computing, cryptography and information processing.
Quantum dots are capable of single and polarization-entangled photon emission and can be easily integrated into photonics circuits.
The strong light - matter interaction that characterizes quantum dots system allows optical control of electron and hole spins states of individual quantum dots, thus enabling quantum gate operations or implementation of different quantum computing protocols.
One of these protocols relies on cluster state, highly entangled quantum states.
Processing of such states, through measurements of single qubits, allows implementation of quantum computing.
This protocol is deemed less demanding than a classic quantum gates approach, which requires multi-qubit input gates.
However, environment noise for single charge carrier is still a limiting factor for any quantum information protocol, because loss of coherence in quantum systems limits the number of operations executable on a qubit.
Recently, GaAs quantum dots produced by droplet etching epitaxy are currently state of the art in terms of low-noise environments for single charge carriers.
In this work, we studied GaAs quantum dots fabricated by a different technique, metalorganic vapour-phase epitaxy (MOVPE) growth on (111)B oriented GaAs substrates pre-patterned by micrometre-scale tetrahedron recesses.
This method enables much higher degree of QD engineering flexibility and site-control of the growth process.
In Chap.1, we introduce the concept of quantum computing and give an overview of a specific implementation, the so called one-way quantum computer, using a specific class of entangled states, cluster states, as main resource for computation.
We will also describe quantum dots, their properties and how they can be used as single and entangled photon emitters.
In Chap.2, we will describe the fabrication steps for producing and processing a particular family of quantum dots, pyramidal site-controlled quantum dots, in order to make them useable for characterization and measurements.
We will also describe the instruments, custom built setups and procedures to optically characterize the quantum dots.
In Chap.3, we investigate several excitation and initialization schemes for quantum dots.
Quantum systems with two or three energy levels are simulated under a variety of optical excitation schemes.
These simulation will define a range of suitable parameters of the optical pulses, compatible the resources available in our laboratory.
Several experimental examples of these excitation scheme will also be provided.
In Chap.4, we carry out an in-depth study of the energetic structure of GaAs quantum dots.
Using a variety of spectroscopic techniques, such as polarization and time resolved micro-photoluminescence, we investigate the optical recombination of several quantum dots to associate to each transition line an excitonic state.
Optical selection rules of our quantum dots have also been investigated, uncovering light-hole-like excited states and deviations from the expected group theory calculations.
In Chap.5, we demonstrate quantum dot initialization by a hole with a well defined spin state, one of the requirements for the physical implementation of quantum computation.
As a step towards cluster state generation, we demonstrate periodic precession of the spin state in a weak magnetic field.
We also attempt cluster state generation, but low quantum dot brightness and fast loss of coherence of the spin state will prove to be very limiting.
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Keywords
Quantum computing , Cluster states , Quantum dots
Citation
Mattana, F. 2025. Advanced micro-photoluminescence spectroscopy of pyramidal site-controlled quantum dots as spin-photon interfaces. PhD Thesis, University College Cork.