Vincent Elfving

A thesis for the degree of Doctor of Philosophy defended December, 2018.

The PhD School of Science, Faculty of Science, Quantum Optics, Niels Bohr Institute, University of Copenhagen

Supervisor:
Anders Søndberg Sørensen

Quantum Information Processing with Quantum Optical Systems

The topic of this thesis is quantum information processing with quantum optical devices. The development progress in this field in recent years can roughly be characterized as twofold: experimental advances in photonics technologies have enabled the creation of strong light-matter interactions between coherent quantum two-level systems and waveguides, while theoretical quantum computing algorithm proposals have started to unlock the potential of quantum information processing architectures. In this thesis, we discuss progress in both of these directions.

Quantum photonics platforms can exhibit exciting single-photon physics with a wide range of interesting applications. To better understand the interactions and physics of singlephoton scattering in waveguides, we apply a recently proposed theoretical scattering framework to specific 1D waveguide scattering problems involving one or two emitters with different level structures. We observe non-trivial interference phenomena on the single-photon level. We further discuss an application to a ground-state entanglement of an emitter with a lambda-type level structure.

We show how sub-radiant states can be engineered in multi-emitter-waveguide devices; we use this physics to our advantage in a new proposal for a quantum information transducer. Such a device can convert a signal between different energy scales. As a device implementation, we consider the transducer acting as a node in a future "quantum internet", where optical photons are the flying information carrier while the stationary qubits operate in the microwave domain. We calculate the device performance in different emitter-waveguide configurations.

We show how engineering the photon-mediated interactions in photonic waveguides can significantly enhance performance and can reach highly efficient transduction and good fidelity, assuming experimentally verified component parameters. As an alternative system, we propose a transducer based on the coupling of an emitter with an optical cavity. By electrically coupling a two-level system to an emitter-cavity system with the same energy splitting, highly efficient transduction can be achieved with good entanglement fidelity. Additionally, we investigate the entanglement fidelity dependence on node dissimilarity and on dephasing effects.

In the last part of this thesis, we discuss how a superconducting quantum computing platform operating with microwave photons can be used to simulate quantum chemistry. Quantum information processing may lead to incredible advances in many fields of science and society as a whole, as the simulation of complex systems previously untouched becomes possible by clever use of quantum resources. We use a dual plane-wave basis approach to solve a 2-dimensional uniform electron gas model on a simulated quantum computer based on superconducting hardware with tunable couplers. We combine different strategies to achieve minimal gate depth and, using a Variational Quantum Eigensolver algorithm, we converge to good fidelity to the groundstate of the problem Hamiltonian. This constitutes a realistic near-term experimental proposal for a practical quantum supremacy experiment on currently available superconducting quantum hardware.