Bound states in the continuum in a fluxonium qutrit

TL;DR

This paper investigates bound states in the continuum (BIC) in a fluxonium qutrit, demonstrating extremely long-lived states with potential for quantum information applications, and analyzing decoherence sources affecting their stability.

Contribution

The study provides a detailed analysis of BICs in fluxonium qutrits, including their lifetimes and decoherence mechanisms, highlighting their potential for long coherence times in superconducting circuits.

Findings

BIC lifetimes can exceed seconds in ideal conditions.

Finite temperature and flux noise significantly impact BIC decay times.

Preparation times for BIC states are on the order of 100 nanoseconds.

Abstract

The heavy fluxonium at zero external flux has a long-lived state when coupled capacitively to any other system. We analyze it by projecting all the fluxonium relevant operators into the qutrit subspace, as this long-lived configuration corresponds to the second excited fluxonium level. This state becomes a bound-state in the continuum (BIC) when the coupling occurs to an extended system supporting a continuum of modes. In the case without noise, we find BIC lifetimes that can be much larger than seconds $T_1\gg {\rm s}$ when the fluxonium is coupled to a superconducting waveguide, while typical device frequencies are in the order of ${\rm GHz}$. We have performed a detailed study of the different sources of decoherence in a realistic experiment, obtaining that upward transitions caused by a finite temperature in the waveguide and decay induced by $1/f$-flux noise are the most dangerous ones. Even in their presence, BICs decay times could reach the range of ${T_1\sim \rm 10^{-1} ms},$ while preparation times are of the order of $10^{2}$ns.