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Fluxonium-Based Artificial Molecule with a Tunable Magnetic Moment

A. Kou*, W. C. Smith, U. Vool, R. T. Brierley, H. Meier, L. Frunzio, S. M. Girvin, L. I. Glazman, and M. H. Devoret

  • Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA

  • *Corresponding author. angela.kou@yale.edu
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Phys. Rev. X 7, 031037 – Published 29 August, 2017

DOI: https://doi.org/10.1103/PhysRevX.7.031037

Abstract

Engineered quantum systems allow us to observe phenomena that are not easily accessible naturally. The LEGO®-like nature of superconducting circuits makes them particularly suited for building and coupling artificial atoms. Here, we introduce an artificial molecule, composed of two strongly coupled fluxonium atoms, which possesses a tunable magnetic moment. Using an applied external flux, one can tune the molecule between two regimes: one in which the ground-excited state manifold has a magnetic dipole moment and one in which the ground-excited state manifold has only a magnetic quadrupole moment. By varying the applied external flux, we find the coherence of the molecule to be limited by local flux noise. The ability to engineer and control artificial molecules paves the way for building more complex circuits for quantum simulation and protected qubits.

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Popular Summary

Nature provides us with a limited set of atoms for constructing useful materials. For detection and characterization of external fields—such as electric or magnetic fields—one would like to have a molecule whose response to the field could change depending on the strength and spatial distribution of the field. Achieving such flexibility, however, is extremely challenging in a molecule synthesized from natural atoms. Superconducting circuits that mimic atoms are a solution to this problem. Such artificial atoms are much less limited in the types of interactions that can be generated between them and their environment. Here, we introduce an artificial molecule whose intrinsic magnetic properties can be changed by an external magnetic field.

Our artificial molecule is composed of two fluxonium atoms (artificial atoms built from Josephson junctions, which are composed of an oxide sandwiched between two layers of superconducting material) coupled via a shared inductance. By applying an external flux, the magnetic moment of the molecule can be tuned in situ from being dipolar to only quadrupolar. We used this tunability to determine that local flux noise is the major source of decoherence in the artificial molecule. In addition, spectroscopic observations of our molecule reveal remarkable agreement between data and a theoretical model with a small number of parameters. This demonstrates that complex superconducting circuits can obey simple Hamiltonians with a few engineerable parameters.

We expect that our experiment will stimulate the creation of novel artificial molecules and materials built using superconducting circuits. Further work on the origins of local flux noise in superconducting circuits is needed, however, before we can harness their full potential.

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Phys. Rev. X 7, 031037– Published 29 August, 2017
  • Received 5 October 2016
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Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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