Contributors: Laboratoire de physique de l'ENS - ENS Paris (LPENS); Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité)-Département de Physique de l'ENS-PSL; École normale supérieure - Paris (ENS-PSL); Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-École normale supérieure - Paris (ENS-PSL); Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL); QUANTum Information Circuits (QUANTIC); Mines Paris - PSL (École nationale supérieure des mines de Paris); Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Sorbonne Université (SU)-Inria de Paris; Institut National de Recherche en Informatique et en Automatique (Inria)-Institut National de Recherche en Informatique et en Automatique (Inria)-Laboratoire de physique de l'ENS - ENS Paris (LPENS); Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité)-Département de Physique de l'ENS-PSL; Université Paris Sciences et Lettres (PSL); Circuits Quantiques Hybrides; Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité)-Département de Physique de l'ENS-PSL; Laboratoire de Physique Théorique et Hautes Energies (LPTHE); Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS); Centre Automatique et Systèmes (CAS); ANR-19-QUAN-0006,QuCoS,Calcul quantique avec les états de chat de Schrödinger(2019); ANR-22-PETQ-0003,RobustSuperQ,Robust superconducting and hybrid quantum bits(2022); ANR-22-PETQ-0006,NISQ2LSQ,From NISQ to LSQ: Bosonic and LDPC codes(2022); European Project: 851740,H2020-EU.1.1. - EXCELLENT SCIENCE - European Research Council (ERC),10.3030/851740,ECLIPSE(2020)
نبذة مختصرة : 12 pages, 10 figures ; Inducing interactions between individual photons is essential for applications in photonic quantum information processing and fundamental research on many-body photon states. A field that is well suited to combine strong interactions and low losses is microwave quantum optics with superconducting circuits. Photons are typically stored in an $LC$ circuit, and interactions appear when the circuit is shunted by a Josephson tunnel junction. Importantly, the zero-point fluctuations of the superconducting phase across the junction control the strength and order of the induced interactions. Superconducting circuits have almost exclusively operated in the regime where phase fluctuations are smaller than unity, and two-photon interactions, known as the Kerr effect, dominate. In this experiment, we shunt a high-impedance $LC$ oscillator by a dipole that only allows pairs of Cooper pairs to tunnel. Phase fluctuations, which are effectively doubled by this pairing, reach the value of 3.4. In this regime of extreme fluctuations, we observe transition frequencies that shift non-monotonically as we climb the anharmonic ladder. From this spectroscopic measurement, we extract two-, three- and four-photon interaction energies of comparable amplitude, and all exceeding the photon loss rate. This work explores a new regime of high-order photon interactions in microwave quantum optics, with applications ranging from multi-photon quantum logic to the study of highly correlated microwave radiation.
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