Viewing protected superconducting qubits through the lens of the cat qubit

Simon Lieu; Emma L. Rosenfeld; Kyungjoo Noh; Connor T. Hann

doi:10.1103/p8fy-qdc8

← Back to search

Viewing protected superconducting qubits through the lens of the cat qubit

Simon Lieu, Emma L. Rosenfeld, Kyungjoo Noh, Connor T. Hann

DOI 10.1103/p8fy-qdc8 · Physical Review A

T1

Active bibliographic source — not scientific approval

Bibliographic access preserves source history; it does not approve extracted materials or validate reported claims. Review warnings on each occurrence separately.

Abstract

We draw analogies between protected superconducting qubits and bosonic qubits by studying the fluxonium Hamiltonian in its Fock basis. The mean-field phase diagram of fluxonium (at the sweet spot) is identified, with a region in parameter space that is characterized by Z2-symmetry-broken ground states. In the heavy fluxonium limit, these ground states are well approximated by squeezed coherent states in a Fock basis (corresponding to persistent current states with definite flux but indefinite charge) and simple expressions are provided for them in terms of the circuit parameters. We study the noise bias in fluxonium via a universal Lindblad master equation and find that the bit-flip rate is exponentially small in Ej/(kBT), while the phase-flip rate does not get worse with this ratio. Analogous behavior is found in cos(2θ) qubits. We describe cat-qubit-inspired bias-preserving X and CX gates for fluxonium. We discuss first steps towards generating an Ising interaction between protected superconducting qubits on a two-dimensional lattice, with the aim of achieving a passive quantum memory by coupling a static Hamiltonian to a generic thermal bath.