Tunable quantum frequency conversion enabled by intermodal nonlinearity in optical fiber

Optical frequency conversion plays a key role in realizing large-scale quantum networks, including multi-qubit discrete-variable quantum computers and quantum communication links where photons serve as the fundamental qubits. However, achieving efficient conversion via nonlinear optical processes for specific target wavelengths remains a significant challenge, as precise dispersion control is essential to satisfy phase-matching conditions across specific frequency ranges. An intriguing approach to solve this challenge is leveraging the modal degree of freedom in spatially multimoded waveguides and realizing intermodal nonlinear interaction. Following this approach, we present the experimental demonstration of tunable, number-state-preserving frequency conversion of true single photons emitted from a quantum dot. The conversion is achieved in a multimode fiber and exhibits a peak internal efficiency of 85% while retaining single photon purity of 99% during conversion. Our results show that the intermodal platform presents a promising and versatile approach for overcoming phase-matching limitations in quantum frequency conversion, thus allowing the efficient interfacing of different optical quantum devices.