Topologically protected quantum bits from Josephson junction arrays

All physical implementations of quantum bits (qubits), carrying the information and computation in a putative quantum computer, have to meet the conflicting requirements of environmental decoupling while remaining manipulable through designed external signals. Protecting qubits from decoherence by exploiting topological stability, a qualitatively new proposal due to Kitaev holds the promise for long decoherence times, but its practical physical implementation has remained unclear so far. Here, we show how a strongly correlated system developing an isolated two-fold degenerate quantum dimer liquid groundstate can be used in the construction of topologically stable qubits. The concrete system we use is the quantum hardcore dimer model studied recently by Moessner and Sondhi, which develops a frustrated dimer liquid ground state with the right properties in a region of the phase diagram. We describe two quantum Josephson junction arrays emulating such a dimer liquid and show how to implement and manipulate qubits. Reference: L. Ioffe et al., Nature 415, 507-510 (2002).