Protected qubits and Chern-Simons theories in Josephson junction arrays

B. Douçot, M. V. Feigel'man, L. B. Ioffe, A. S. Ioselevich

Research output: Contribution to journalArticlepeer-review

141 Citations (Scopus)


We present general symmetry arguments that show the appearance of doubly degenerate states protected from external perturbations in a wide class of Hamiltonians. We construct the simplest spin Hamiltonian belonging to this class and study its properties both analytically and numerically. We find that this model generally has a number of low energy modes which might destroy the protection in the thermodynamic limit. These modes are qualitatively different from the usual gapless excitations as their number scales as the linear size (instead of volume) of the system. We show that the Hamiltonians with this symmetry can be physically implemented in Josephson junction arrays and that in these arrays one can eliminate the low energy modes with a proper boundary condition. We argue that these arrays provide fault tolerant quantum bits. Further we show that the simplest spin model with this symmetry can be mapped to a very special Z 2 Chern-Simons model on the square lattice. We argue that appearance of the low energy modes and the protected degeneracy is a natural property of lattice Chern-Simons theories. Finally, we discuss a general formalism for the construction of discrete Chern-Simons theories on a lattice.

Original languageEnglish
Article number024505
JournalPhysical Review B - Condensed Matter and Materials Physics
Issue number2
Publication statusPublished - Jan 2005
Externally publishedYes


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