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 Magnifying quantum phase fluctuations with Cooper-Pair pairing


Zaki Legthas (LPENS Paris)


Horaire/Time : Tuesday, 4th April at 14:00


Lieu/Place : Visio-conférence Zoom hydride depuis Salle Rémy Lemaire K223, Institut Néel

Zoom link : https://univ-grenoble-alpes-fr.zoom.us/j/91808901596?pwd=UWZ2cml2N1VBOEZBenk0d3RJek9rdz09


Résumé/Abstract : Remarkably, complex assemblies of superconducting wires, electrodes, and Josephson junctions are compactly described by a handful of collective phase degrees of freedom that behave like quantum particles in a potential. Almost all these circuits operate in the regime where quantum phase fluctuations are small—the associated flux is smaller than the superconducting flux quantum—although entering the regime of large fluctuations would have profound implications for metrology and qubit protection. The difficulty arises from the apparent need for circuit impedances vastly exceeding the resistance quantum. Independently, exotic circuit elements that require Cooper pairs to form pairs in order to tunnel have been developed to encode and topologically protect quantum information. In this work, we demonstrate that pairing Cooper pairs magnifies the phase fluctuations of the circuit ground state. In a first experiment, we measure a tenfold suppression of flux sensitivity of the first transition energy only, implying a twofold increase in the vacuum phase fluctuations and showing that the ground state is delocalized over several Josephson wells. In a second experiment, we demonstrate that Cooper-pair pairing mediates high order photon-photon interactions, resulting in some peculiar spectral properties.

Ref.: W. C. Smith et al., Phys. Rev. X 12, 021002