  Flux qubits, small superconducting loops interrupted by Josephson junctions,
are successful realizations of quantum coherence for macroscopic variables.
Superconductivity in these loops is carried by $\sim 10^6$ -- $10^{10}$
electrons, which has been interpreted as suggesting that coherent
superpositions of such current states are macroscopic superpositions analogous
to Schr\"odinger's cat. We provide a full microscopic analysis of such qubits,
from which the macroscopic quantum description can be derived. This reveals
that the number of microscopic constituents participating in superposition
states for experimentally accessible flux qubits is surprisingly but not
trivially small. The combination of this relatively small size with large
differences between macroscopic observables in the two branches is seen to
result from the Fermi statistics of the electrons and the large disparity
between the values of superfluid and Fermi velocity in these systems.

Quantum Physics;Superconductivity