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arXiv · 2312.15075

Spectral signature of high-order photon processes enhanced by Cooper-pair pairing

W. C. Smith·A. Borgognoni·M. Villiers·E. Roverc'h·J. Palomo·M. R. Delbecq·T. Kontos·P. Campagne-Ibarcq·B. Douçot·Z. Leghtas
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Abstract

Inducing interactions between individual photons is key for photonic quantum information and studying many-body photon states. Superconducting circuits are well suited to combine strong interactions with low losses. Typically, microwave photons are stored in an $LC$ oscillator shunted by a Josephson junction, where the zero-point phase fluctuations across the junction determine the strength and order of photon interactions. Most superconducting nonlinear oscillators operate with small phase fluctuations, where two-photon Kerr interactions dominate. In our experiment, we shunt a high-impedance $LC$ oscillator with a dipole element favoring the tunneling of paired Cooper pairs. This leads to large phase fluctuations of 3.4, accessing a regime where transition frequencies shift non-monotonically with excitation number. From spectroscopy, we extract two-, three-, and four-photon interaction energies, all of similar strength and exceeding the photon loss rate. Our results open a new regime of high-order photon interactions in microwave quantum optics.

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W. C. Smith, A. Borgognoni, M. Villiers, E. Roverc'h, J. Palomo, M. R. Delbecq, T. Kontos, P. Campagne-Ibarcq, B. Douçot, Z. Leghtas. 2023-12-22. Spectral signature of high-order photon processes enhanced by Cooper-pair pairing. https://doi.org/10.1038/s41467-025-62047-8

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