Thermal noise is ubiquitous in microscopic systems and high-precision measurements. The control of thermal noise would reveal quantum regimes1 and enable fundamental physics searches2. Recently, nonlinearity in microresonators has enabled laser devices such as Kerr microresonator soliton frequency combs3. Soliton microcombs explore nonlinear dynamics and enable optical synthesizers4, optical clockwork5 and data communications systems6. Here, we explore how thermal noise leads to the fundamental decoherence of microcombs. We show that a particle-like soliton, which is an ensemble of comb modes, is closely coupled to the thermal fluctuations of its silicon-chip-based resonator. The microcomb modal linewidth is thus thermally broadened, and we characterize these thermal-noise correlations through a soliton effective temperature. Moreover, we demonstrate that passive laser cooling reduces soliton thermal decoherence to far below the ambient-temperature limit. We implement laser cooling by photothermal forcing, and we observe cooling of the frequency comb modes to 84 K. Our work illuminates inherent connections between nonlinear photonics and microscopic fluctuations.
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We thank K. Srinivasan for fabricating the SiN microresonators, S.-P. Yu for creating the mode simulation in Fig. 1, D. Spencer for experimental assistance, and Srico, Inc. for the use of the periodically poled lithium-niobate waveguide device. This research is supported by the Defense Advanced Research Projects Agency DODOS programme, AFOSR (FA9550-16-1-0016), NRC and NIST.
The authors declare no competing interests.
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Drake, T.E., Stone, J.R., Briles, T.C. et al. Thermal decoherence and laser cooling of Kerr microresonator solitons. Nat. Photonics 14, 480–485 (2020). https://doi.org/10.1038/s41566-020-0651-8
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