Generating Optical Graph States

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


Quantum technologies promise to revolutionise the fields of computation, communication and sensing, but require precise control over large ensembles of quantum systems---a formidable challenge. Quantum computing, in particular, requires potentially millions of qubits able to interact in arbitrary combination. Graph states---the predominant language of qubit entanglement---enable the prevailing, measurement-based model of quantum computation, which increases scalability by reducing qubit interaction requirements to nearest neighbours. Meanwhile, optics has long been the test bed for quantum phenomena. Meanwhile, integrated optics contends in the race to build practical quantum computers, incorporating high-performance components with massive scalability. The purpose of the thesis is to develop multiphoton capability in integrated optics for generating graph states, on the road to linear optical quantum computing. To do so, I will investigate current methods for generating graph states theoretically, numerically and experimentally. In Chapter 2, I derive rules for the successful postselection of optical graph states, bringing to light severe limitations to the technique. I then combine these rules with Monte-Carlo numerics to learn which graph states are accessible to every type of single photon source.I also identify optimal interferometers for the generation of graph states up to $8$ qubits which are feasible today. Then, in Chapters 3 and 4, I report on the first integrated device to generate an entangled state of four-photons. The programmable chip generates, for the first time, both kinds of four-qubit graph state, and breaks the multiphoton barrier for integrated optics. Further, the device demonstrates high-visibility heralded Hong-Ou-Mandel interference. Finally, in Chapter 5, I use local complementaion---which traverses all locally equivalent graph states---to generate the orbits of every entanglement class of $n<10$ qubits. These achievements light the way to scalable graph state generation with photons, and eventually linear optical quantum computing.
Date of Award7 May 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorJohn G Rarity (Supervisor), Mark Thompson (Supervisor) & Joshua W Silverstone (Supervisor)


  • quantum optics
  • quantum computing
  • quantum computing architectures
  • integrated optics
  • silicon photonics
  • photonics
  • graph states

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