Bound states in the continuum in a fluxonium qutrit

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 T1 that can be much larger than seconds when the fluxonium is coupled to a superconducting waveguide, while typical device frequencies are on the order of gigahertz. We have performed a detailed study of the different sources of decoherence in a realistic experiment, finding that upwards 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, BIC decay times could reach the range of T1 ti 10-1 ms, while preparation times are of the order of 102 ns.

Automated summary

The study analyzes the coupling between heavy fluxonium without external flux and other systems and finds that when coupled to an extended system supporting a continuum of modes, the fluxonium can form a long-lived state. In the absence of noise, coupling the fluxonium to a superconducting waveguide can result in lifetimes on the order of seconds for the bound states in the continuum (BIC), even though typical device frequencies are in the gigahertz range. The presence of decoherence sources, such as finite temperature and 1/f flux noise, can affect the BIC decay times in realistic experiments.