Hybrid photonic circuits for fundamental quantum physics

In the course of this project, we will develop a novel experimental platform to test the foundations of quantum physics with single photons, that is, single particles of light. Specifically, we aim to increase the accuracy of two particular tests, which probe the nature of the quantum wave function and its relation to measurable probabilities, by one order of magnitude over previous implementations. The two key ingredients, which will make this possible, are bright sources of single photons with stable and controllable emission properties as well as compact waveguide interferometers with high overall transmission efficiency, excellent stability and accurately adjustable transmission of the individual interferometer arms. As a single-photon sources, we will employ hexagonal boron nitride (hBN), which emits photons at a wavelength of 575 nm in the yellow visible spectrum. They allow for a controlled emission of single photons at adjustable repetition rate with very good suppression of undesired multi-photon emissions. Most crucially, the emission works at room temperature, such that no expensive and bulky cryogenic setups will be required to operate our platform. The adjustable repetition rate will help to minimise systematic errors from nonlinearities in the single-photon detection systems. To develop the waveguide interferometer, we will utilize the latest advances in lithium niobate-on- insulator (LNOI) integrated photonics technology. Our aim is to develop interferometers, which are switched by heating individual waveguides, at a contrast of 1:100 000 between their on and off-states and at the same time minimising cross-talk between the waveguide channels. Both systems, hBN emitters and LNOI interferometers, will be integrated in a single photonic circuit, which will be a technical innovation in itself. A further development will be on-chip conversion from the hBN emission wavelength at 575 nm to the telecom C-band (1550 nm), which is advantageous for the interferometer design and is also the ideal wavelength for long-distance quantum communication and networking, due to minimal propagation losses in optical fibers. As outcomes of the project, we anticipate an experimental test of the foundations of quantum physics with record accuracy, thereby narrowing the constraints on generalised quantum theories. In addition, the project results will provide a technology benchmark for the novel hybrid hBN-LNOI platform, which we will develop in the course of the project. Due to the seamless and efficient interfacing of a bright and pure room-temperature single-photon emitter with a waveguide system capable of high-contrast electro-optic switching and flexible, efficient on-chip wavelength conversion, we expect a broad application potential of the technology in the fields of quantum communication and photonic quantum computation.