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Résumé : Superconducting quantum devices usually operate Josephson junctions in the superconducting state as nonlinear inductors to form anharmonic oscillators. Strong anharmonicity is used for qubits while weak anharmonicity in used for amplification to parametrically convert pump photons into signal and idler photons. In this talk I will show that Josephson junctions can also yield useful quantum devices when operated in the voltage state. In this configuration, called Josephson photonics, Cooper pairs, tunneling through a junction biased at a voltage V below the gap, can transfer their energy 2eV into one or more photons in the harmonic modes of the circuit in which the junction is embedded (1). In Josephson photonics, the junction, therefore, acts as a nonlinear drive, doing away with the pump tones that are usually required to operate quantum devices, thereby removing the hardware overhead required for generating, routing, and filtering of these tones. I will show how we use Josephson photonics to implement quantum-limited linear amplifiers (2) and photon-number amplifiers (3), which amplify photon number while deamplifying phase and promise to be useful for counting itinerant microwave photons. (1) Hofheinz et al., Phys. Rev. Lett. 106, 217005 (2011) (2) Martel et al., Appl. Phys. Lett. 126, 074001 (2025) (3) Albert et al., Phys. Rev. X 14, 011011 (2024)
