( But ( , ( ( if ( ( the ( bias voltage ) ) ( is ( raised ( to ( the ( gap voltage ) ) ) ) ) ) ) ( ( ( the flow ) ( of ( quasi-particles ( across ( the junction ) ) ) ) ) ( ( ( becomes possible ) ( because ( ( the energy ) ( ( gained ( across ( ( the drop ) ( of ( electrical ( potential compensates ) ) ) ) ) ) ( for ( ( the energy ) ( spent ( in ( breaking ( a ( Cooper pair ) ) ) ) ) ) ) ) ) ) ) . ) ) ) ) )
(ROOT (S (CC But) (, ,) (SBAR (IN if) (S (NP (DT the) (NN bias) (NN voltage)) (VP (VBZ is) (VP (VBN raised) (PP (TO to) (NP (DT the) (NN gap) (NN voltage))))))) (NP (NP (DT the) (NN flow)) (PP (IN of) (NP (NP (NNS quasi-particles)) (PP (IN across) (NP (DT the) (NN junction)))))) (VP (VBZ becomes) (ADJP (JJ possible)) (SBAR (IN because) (S (NP (DT the) (NN energy)) (VP (VBD gained) (PP (IN across) (NP (NP (DT the) (NN drop)) (PP (IN of) (NP (JJ electrical) (JJ potential) (NNS compensates))))) (PP (IN for) (NP (NP (DT the) (NN energy)) (VP (VBN spent) (PP (IN in) (S (VP (VBG breaking) (NP (DT a) (NNP Cooper) (NN pair)))))))))))) (. .)))
But, if the bias voltage is raised to the gap voltage the flow of quasi-particles across the junction becomes possible because the energy gained across the drop of electrical potential compensates for the energy spent in breaking a Cooper pair.
(ROOT (S (NP (PRP We)) (VP (VBP use) (ADJP (JJ voltage) (S (VP (TO to) (VP (VB keep) (NP (NP (NN track)) (PP (IN of) (NP (JJ electric) (JJ potential) (NN energy))))))))) (. .)))
We use voltage to keep track of electric potential energy.