Theory of 4e versus 2e supercurrent in frustrated Josepshon-junction   rhombi chain
  We consider a chain of Josepshon-junction rhombi (proposed originally in \cite{Doucot}) in quantum regime, and in the realistic case when charging effects are determined by junction capacitances. In the maximally frustrated case when magnetic flux through each rhombi $\Phi_r$ is equal to one half of superconductive flux quantum $\Phi_0$, Josepshon current is due to correlated transport of {\em pairs of Cooper pairs}, i.e. charge is quantized in units of $4e$. Sufficiently strong deviation $ \delta\Phi \equiv |\Phi_r-\Phi_0/2| > \delta\Phi^c$ from the maximally frustrated point brings the system back to usual $2e$-quantized supercurrent. We present detailed analysis of Josepshon current in the fluctuation-dominated regime (sufficiently long chains) as function of the chain length, $E_J/E_C$ ratio and flux deviation $ \delta\Phi$. We provide estimates for the set of parameters optimized for the observation of $4e$-supercurrent. 
Ivan V. Protopopov and Mikhail V. Feigel'man