Information transfer in open quantum systems

This thesis covers open quantum systems and information transfer in the face of dissipation and disorder through numerical simulation. In Chapter 3 we present work on an open quantum system comprising a two-level system, single bosonic mode and dissipative environment; we have included the bosonic mode in the exact system treatment. This model allows us to gain an understanding of an environment’s role in small energy transfer systems. We observe how the two-level system-mode coupling strength and the spectral density form characterising the environment interplay, affecting the system’s coherent behaviour. We find strong coupling and a spectral density resonantly peaked on the two-level system oscillation frequency enhances the system’s coherent oscillatory dynamics. Chapter 4 focusses on a physically motivated study of chain and ladder spin geometries and their use for entanglement transfer between qubits. We consider a nitrogen vacancy centre qubit implementation with nitrogen impurity spin-channels and demonstrate how matrix product operator techniques can be used in simulations of this physical system. We investigate coupling parameters and environmental decay rates with respect to transfer efficiency effects. Then, in turn, we simulate the effects of missing channel spins and disorder in the spin-spin coupling. We conclude by highlighting where our considered channel geometries outperform each other. The work in Chapter 5 is an investigation into the feasibility of routing entanglement between distant qubits in 2D spin networks. We no longer consider a physical implementation, but keep in mind the effects of dissipative environments on entanglement transfer systems. Starting with a single sending qubit-ancilla and multiple addressable receivers, we show it is possible to target a specific receiver and establish transferred entanglement between it and the sender’s ancilla through eigenstate tunnelling techniques. We proceed to show that eigenstate tunnelling-mediated entanglement transfer can be achieved simultaneously from two senders across one spin network

Coherent exciton dynamics in a dissipative environment maintained by an off-resonant vibrational mode

This work was supported by the Leverhulme Trust (RPG-080) and the EPSRC (EP/G03673X/1).The interplay between an open quantum system and its environment can lead to both coherent and incoherent behavior. We explore the extent to which strong coupling to a single bosonic mode can alter the coherence properties of a two-level system in a structured environment. This mode is treated exactly, with the rest of the environment comprising a Markovian bath of bosonic modes. The strength of the coupling between the two-level system and the single mode is varied for a variety of forms for the bath spectral density in order to assess whether the coherent dynamics of the two-level system are modified. We find a clear renormalization of the site population oscillation frequency that causes an altered interaction with the bath. This leads to enhanced or reduced coherent behavior of the two-level system, depending on the form of the spectral density function. We present an intuitive interpretation, based on an analytical model, to explain the behavior.Publisher PDFPeer reviewe

Designing spin-channel geometries for entanglement distribution

We investigate different geometries of spin-1/2 nitrogen impurity channels for distributing entanglement between pairs of remote nitrogen vacancy centers (NVs) in diamond. To go beyond the system size limits imposed by directly solving the master equation, we implement a matrix product operator method to describe the open system dynamics. In so doing, we provide an early demonstration of how the time-evolving block decimation algorithm can be used for answering a problem related to a real physical system that could not be accessed by other methods. For a fixed NV separation there is an interplay between incoherent impurity spin decay and coherent entanglement transfer: Long-transfer-time, few-spin systems experience strong dephasing that can be overcome by increasing the number of spins in the channel. We examine how missing spins and disorder in the coupling strengths affect the dynamics, finding that in some regimes a spin ladder is a more effective conduit for information than a single-spin chain.Publisher PDFPeer reviewe

Coherent exciton dynamics in a dissipative environment maintained by an off-resonant vibrational mode

Dataset to support article for Phys Rev A 93 042109 (2016) &quot;Coherent exciton dynamics in a dissipative environment maintained by an off-resonant vibrational mode&quot; 2</span

Data underpinning - Coherent exciton dynamics in a dissipative environment maintained by an off-resonant vibrational mode (Phys Rev A 93 042109 (2016))

Data for Phys Rev A 93 042109 (2016