Quantum-interference metrology of dissipative Kerr solitons

Dissipative Kerr solitons in optical microresonators underpin chip-scale frequency combs with applications ranging from coherent telecommunications to precision spectroscopy. Yet the characterization of their intrinsic femtosecond temporal structure remains challenging, as the low pulse energy and broad spectral bandwidth necessitate optical amplification and careful dispersion compensation in conventional ultrafast diagnostics, both of which can substantial distort the waveform. Here, we demonstrate a quantum-interference metrology of microcomb solitons based on Hong-Ou-Mandel interference. By attenuating the soliton stream to the single-photon level and measuring fourth-order interference, we directly retrieve near transform-limited pulse durations without amplification or dispersion management, remaining accurate even after propagation through 25 kilometers of standard fiber. The same interferogram also provides direct access to the temporal separations in multisoliton states by converting intersoliton separations into additional interference dips at corresponding delays, enabling subpicosecond characterization of their intracavity temporal structure. This quantum-inspired paradigm introduces a metrological approach that is immune to amplification and dispersion distortions, offering a powerful tool for the characterization of complex soliton physics.