Controlling dephasing of coupled qubits via shared-bath coherence

Hall, Luke 2025. Controlling dephasing of coupled qubits via shared-bath coherence. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Quantum dots are nanoscale semiconductor crystals in which charge carriers are confined in all three spatial dimensions. The discrete energy levels of quantum dots make them promising candidates for qubits in quantum information processing. However, at any temperature, there are lattice vibrations, known as phonons, in the surrounding material. When a quantum dot is optically excited, an exciton, a bound electron-hole pair, is created and there is an unavoidable coupling of the exciton to the phonons. The interaction of a quantum system with these phonons causes decoherence, limiting the coherence time, which restricts the utility of qubits in quantum information processing applications. In this thesis, the decoherence in a system of two spatially separated, electronically decoupled qubits, with direct or mediated coupling, interacting with a shared three-dimensional bath is investigated. For illustration, Förster or cavity-mediated coupling between semiconductor quantum dots interacting with acoustic phonons is treated. Using the rigorous Trotter decomposition with linked cluster expansion technique, a reduction in decoherence at specific distances between the quantum dots is observed. This reduction results from the collective coupling of the qubits to shared phonon modes, enabled by the coherent properties of the bath. In particular, when the qubit separation is an integer multiple of the phonon wavelength, selected by the energy splitting of the coupled qubit states, there is a reduction in the decoherence. We show that a near-vanishing dephasing rate, which can be referred to as a one-dimensional regime, can be achieved by utilising strong quantum dot-cavity coupling strengths. We quantify the separations at which the one-dimensional-like regime persists before transitioning to the expected three-dimensional behaviour. To calculate the dynamics of these extended quantum systems, traditional path-integral based tensor-multiplication schemes are not sufficient. To tackle this, an optimisation scheme is developed, using a matrix representation of tensors and their singular value decomposition to filter out unimportant contributions. Importantly, more memory-efficient representations for the tensors exist; however, this approach enables the usage of an extrapolation scheme which approximates the exact long-time dynamics. This optimisation dramatically reduces both computational time and memory usage compared with the traditional methods.

Item Type:	Thesis (PhD)
Status:	Unpublished
Schools:	Schools > Physics and Astronomy
Subjects:	Q Science > QC Physics
Uncontrolled Keywords:	Non-Markovian dynamics; open quantum systems; quantum dots; qubits; exciton–phonon interactions; tensor networks; path integral methods; quantum coherence; decoherence
Funders:	EPSRC
Date of First Compliant Deposit:	8 September 2025
Last Modified:	09 Sep 2025 09:27
URI:	https://orca.cardiff.ac.uk/id/eprint/180960

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