On Quantum Rainbows: Density Operator in the Frequency-Bin Representation for Entangled Twin-Photons Generated With Sub-Threshold Microcombs

• Published in: IEEE journal of selected topics in quantum electronics 2024-09, Vol.30 (5: Microresonator Frequency Comb Technologies), p.1-8
• Main Authors: Liu, Fengyu, Chembo, Yanne K.
• Format: Article
• Language: English
• Subjects: Correlation; Density; Four-wave mixing; frequency-bin entanglement; Kerr optical frequency combs; Laser excitation; Microcavities; Optical frequency; Performance measurement; Photonics; Photons; Pump lasers; Resonant frequency; Resonators; Room temperature; twin-photon generation; Whispering-gallery mode resonators
• ISSN: 1077-260X; 1558-4542
• DOI: 10.1109/JSTQE.2024.3402679

Kerr optical frequency combs are generated by pumping a high-Q integrated microresonator with a resonant laser. Below threshold, the pump laser field mediates the phenomenon of spontaneous four-wave mixing, where two pump photons are symmetrically up- and down-converted as twin photons that can be entangled across up to tens of eigenmodes in the spectral domain. While these room-temperature integrated photonic circuits are expected to play a central role in quantum technology, their high dimensionality and dissipative nature are a challenge for their theoretical description, therefore hindering the understanding of their properties and potential of performance. In this article, we develop a framework that permits to obtain an explicit solution for the density operator of quantum microcombs below threshold. This self-consistent theoretical description allows for their complete characterization, as well as for the analytical determination of various performance metrics such as fidelity, purity, and entropy.