Laser written integrated photonics for quantum information science
thesisposted on 28.03.2022, 12:20 by Thomas Meany
This thesis details the study of quantum information science (QIS) using integrated photonics. Integrated photonic devices are fabricated in glass using the femtosecond laser direct write (FLDW) technique. This method uses a focused high power laser to produce a localised refractive index change in a glass substrate which can be used to form waveguides. A rigorous parameter study of laser inscription and glass structure is performed to isolate regions where low loss waveguides can be formed. Unique, three dimensional,circuit designs are created which are then characterised to determine symmetry and to ascertain their performance for QIS. The circuit designs include 3D multiports which permit the unitary transformation of a set of optical modes. Single photons are injected into this device to determine its performance and compare it to bulk optic, fibre optic and lithographically fabricated examples. The 3D multiports show high fidelity operation and a comparable performance to other circuit design platforms. Building on this work which shows the high quality of laser inscribed devices, an inherently quantum circuit is designed. It has the function of operating as a basic two-qubit circuit element which applies a phase shift to a qubit in a target mode, conditional on the state of a control qubit. Thiscircuit is heralded, meaning that it operates in the presence of two additional ancilla modes which trigger the success of the probabilistic gate. The design of this circuit required detailed analysis of the reproducibility of laser written circuits in the presence of performance tolerance to fabrication imperfections. The devices described previously were characterised, non-classically, using a bulk source of photon pairs. This limits the application of the devices beyond demonstrations or prototypes, hence it is desirable to also integrate these devices with on-chip sources of single photons. Such a source of single photons is available in the form of a quasi phase-matched nonlinear crystal which emits heralded single photons. An experiment was undertaken to design a hybrid circuit, composed of both linear and nonlinear elements to produce heralded single photons, to produce multiple sources of heralded single photons. This was completed and experiments exploiting high speed switching to combine individual sources and an experiment to manipulate photon pair states is completed. This work builds on the knowledge of FLDW structures for on chip routing and manipulation of light. Demonstrations of integrated circuits and hybrid integrated devices shows the potential for high quality and compact monolithic on-chip quantum circuits.