Coulomb-blockade-induced bound quasiparticle states in a double-island qubit

We study the low temperature thermodynamic properties of a superconducting double-island qubit. For an odd number of electrons in the device, the ground state corresponds to the intrinsic quasiparticle bound to the tunneling contact. The ground state is separated from the continuum of excited states by a finite gap of order of the Josephson energy E_J. The presence of the bound quasiparticle state results in a nonmonotonic temperature dependence of the width of the transition region between Coulomb blockade plateaus. The minimum width occurs at the ionization temperature of the bound state, T_i\sim E_J/\ln(\sqrt{\Delta E_J}/\delta), with \Delta and \delta being respectively the superconducting gap and single particle mean level spacing in the island. For an even number of electrons in the system, we show that the Coulomb enhancement of the Josephson energy can be significantly stronger than that in the case of a single grain coupled to a superconducting lead. If the electrostatic energy favors a single broken Cooper pair, the resulting quasiparticles are bound to the contact with an energy that is exponentially small in the inverse dimensionless conductance.