On-chip tunable quantum interference in a lithium niobate-on-insulator photonic integrated circuit

Abstract Programmable interferometric circuits are at the heart of integrated quantum photonic processors. While the lithium niobate-on-insulator platform has the potential to advance integrated quantum photonics due to its strong nonlinearity and tight mode confinement, the demonstration of reconfigurable two-photon interference has not yet been achieved. Here, we design, fabricate and characterize the building block of such interferometric networks in the form of a 2x2 Mach-Zehnder Interferometer. We use a thermo-optic phase shifter to achieve stable performance with a power consumption of Pπ = 44.4 mW and sub-microsecond switching times. We demonstrate the effectiveness of our device for quantum applications by measuring single-photon routing with up to 34 dB extinction ratio. We show Hong-Ou-Mandel interference with fully tunable visibilities reaching a maximum value of 97.4 ± 1.0 %. As part of large scale quantum photonic circuits, this building block will facilitate reconfigurable and tunable photonic processing units integrated alongside non-classical light sources.