  Recently a one-dimensional closed ladder of Josephson junctions has been
studied (G. Cristofano et al., Phys. Lett. A 372 (2008) 2464) within a twisted
conformal field theory (CFT) approach (G. Cristofano et al., Mod. Phys. Lett. A
15 (2000) 1679; Nucl. Phys. B 641 (2002) 547) and shown to develop the
phenomenon of flux fractionalization (G. Cristofano et al., Eur. Phys. J. B 49
(2006) 83). That led us to predict the emergence of a topological order in such
a system (G. Cristofano et al., JSTAT (2005) P03006). In this letter we analyze
the ground states and the topological properties of fully frustrated Josephson
junction arrays (JJA) arranged in a Corbino disk geometry for a variety of
boundary conditions. In particular minimal configurations of fully frustrated
JJA are considered and shown to exhibit the properties needed in order to build
up a solid state qubit, protected from decoherence. The stability and
transformation properties of the ground states of the JJA under adiabatic
magnetic flux changes are analyzed in detail in order to provide a tool for the
manipulation of the proposed qubit.
