Hybrid photonic circuits for fundamental quantum physics

• Applicant: Professor Dr. Tobias Vogl
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 550846026

Quantum mechanics as a physical theory rests on a set of fundamental postulates. Two of these, the complex-number representation of the wavefunction as well as the Born rule can be tested with single quanta in multi-path interferometers with switchable transmission of its individual paths. The accuracy and precision of these tests are governed by the stability and throughput of the interferometers as well as on the control over other error sources related to the path transmission switching characteristics and nonlinearities in the experiment. Two experimental ingredients, which have individually been proven to deliver the desired properties in the realm of photons, are integrated waveguide interferometers and bright, controllable single photon sources. We aim to combine these two ingredients and test the two postulates of quantum mechanics with increased accuracy (improving by one order of magnitude over the state-of-the-art) in a single experiment. This will become possible by combining the latest advances in hexagonal boron nitride (hBN) single photon emitters with lithium niobate-on-insulator (LNOI) integrated photonics technology in a single platform. We will develop and fabricate LNOI waveguide interferometers with 50 dB extinction ratio thermo-optic switching and minimized inter-channel crosstalk. These interferometers will be integrated with hBN emitters in a hybrid photonic circuit, aiming for 20% coupling efficiency or better. A further development will be on-chip frequency conversion from the natural 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. The hBN emitters will be excited under conditions, which minimize errors from single photon detector nonlinearity. We anticipate the outcomes of the project to test the foundations of quantum mechanics with record accuracy, thereby narrowing the constraints on generalized quantum theories. In addition, they 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 extinction-ratio switching and flexible, efficient on-chip frequency conversion, we expect a broad application potential of the technology in quantum communication and photonic quantum computation.

• DFG Programme: Research Grants
• International Connection: Austria, Switzerland
• Partner Organisation: Schweizerischer Nationalfonds (SNF)
• Cooperation Partners: Professorin Dr. Rachel Grange; Dr. Robert Keil