  From a physicist's standpoint, the most interesting part of quantum computing
research may well be the possibility to probe the boundary between the quantum
and the classical worlds. The more macroscopic are the structures involved, the
better. So far, the most "macroscopic" qubit prototypes that have been studied
in the laboratory are certain kinds of superconducting qubits. To get a feeling
for how macroscopic these systems can be, the states of flux qubits which are
brought in a quantum superposition corresponds to currents composed of as much
as 10^5-10^6 electrons flowing in opposite directions in a superconducting
loop. Non-superconducting qubits realized so far are all essentially
microscopic.
