Quantum Phase Transitions and Particle-Hole Pair Transport in   Capacitively Coupled Josephson-Junction Chains
  We consider two chains of ultrasmall Josephson junctions, coupled capacitively with each other, and investigate the transport of particle-hole pairs and the quantum phase transitions at zero temperature. For appropriate parameter ranges, the particle-hole pairs are found to play major roles in transport phenomena; condensation of such pairs leads to the superconducting state, displaying perfect drag of supercurrents along the two chains. 
Mahn-Soo Choi (POSTECH), M. Y. Choi (SNU), and Sung-Ik Lee (POSTECH)