If the Cooper box is sufficiently small, e.g., as small as a quantum dot, the charging current breaks into discrete transfer of individual Cooper pairs, so that ultimately it is possible to just transfer a single Cooper pair across the junction. The effectiveness of the Cooper pair transfer depends on the energy difference between the box and the bulk and a maximum is reached when a voltage is applied, which equalizes this energy difference. Unlike Loss and Vincenzo's computer, this one actually exists in the laboratory. Nakamura, Pashkin and Tsai did not perform any computations with it though. At this stage it was enough of an art to observe the coherence for about 6 cycles of the Cooper pair oscillations, while the chip was cooled to about and carefully shielded from external electromagnetic radiation.
Further progress is likely to be difficult.
Tsai was more cautious, but emphasized the importance of having mastered the control of quantum states in the Cooper pair box.