Magnifying quantum phase fluctuations with Cooper-pair pairing

TL;DR

This paper demonstrates a method to magnify quantum phase fluctuations in superconducting circuits by tuning a Josephson element between single- and two-Cooper-pair tunneling, enhancing the probing of quantum states.

Contribution

The authors introduce a tunable Josephson element that doubles the potential corrugation frequency, significantly amplifying quantum phase fluctuations in superconducting circuits.

Findings

Tenfold suppression of flux sensitivity of the first transition energy

Twofold increase in vacuum phase fluctuations

Enhanced probing of quantum phase delocalization

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. The inductive wires contribute a parabolic confinement, while the tunnel junctions add a cosinusoidal corrugation. Usually, the ground state wavefunction is localized within a single potential well -- that is, quantum phase fluctuations are small -- although entering the regime of delocalization holds promise for metrology and qubit protection. A direct route is to loosen the inductive confinement and let the ground state phase spread over multiple Josephson periods, but this requires a circuit impedance vastly exceeding the resistance quantum and constitutes an ongoing experimental challenge. Here we take a complementary approach and fabricate a generalized Josephson element that can be tuned in situ between one- and two-Cooper-pair tunneling, doubling the frequency of the corrugation and thereby magnifying the number of wells probed by the ground state. We measure a tenfold suppression of flux sensitivity of the first transition energy, implying a twofold increase in the vacuum phase fluctuations.