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Laser Resonators, Microresonators, and Beam Control

Advances in quantum microcombs

Fengyu Liu, Yanne K. Chembo

Published Mar 12, 2024• 0 citations
Publisher

Abstract

Quantum microcombs are nonclassical states of light generated when twin-photons are excited via spontaneous four-wave mixing in a microresonator. In general, intrinsic losses are overlooked and the theoretical description of these systems implicitly assumes that the quantum microcombs are pure states. In this communication, we propose a density operator theoretical framework that is valid in the general case of a lossy resonator.

DOI
10.1117/12.3002859

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Quantum microcombs are nonclassical states of light generated when twin-photons are excited via spontaneous four-wave mixing in a microresonator. In general, intrinsic losses are overlooked and the theoretical description of these systems implicitly assumes that the quantum microcombs are pure states. In this communication, we propose a density operator theoretical framework that is valid in the general case of a lossy resonator.
Kerr optical frequency combs are generated by pumping a high-QQQintegrated 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.
Quantum Kerr optical frequency combs are generated by pumping a high-Q microresonator with a continuous wave resonant laser. Below threshold, two pump photons are symmetrically up- and down-converted as twin photons via spontaneous four-wave mixing, thereby generating up to a hundred paired channels. Furthermore, these chipscale, high-dimensional and room-temperature systems are expected to play a major role in quantum engineering. However, their theoretical description is still unclear. Here, we present an explicit description of this quantum system via a frequency-bin state and its density operator, and explore the properties of the eigenkets.
An optical frequency comb comprises a cluster of equally spaced, phase-locked spectral lines. Replacing these classical components with correlated quantum light gives rise to cluster quantum frequency combs, providing abundant quantum resources for measurement-based quantum computation, and multi-user quantum networks. We propose and generate cluster quantum microcombs within an on-chip optical microresonator driven by multi-frequency lasers. Through resonantly enhanced four-wave mixing processes, continuous-variable cluster states with 60 qumodes are deterministically created. The graph structures can be programmed into one- and two-dimensional lattices by adjusting the configurations of the pump lines, which are confirmed inseparable based on the measured covariance matrices. Our work demonstrates the largest-scale cluster states with unprecedented raw squeezing levels from a photonic chip, offering a compact and scalable platform for computational and communicational tasks with quantum advantages.