In this paper, the degenerate ground states of Z2 topological order on a plane with holes (the so-called surface codes) are used as the protected code subspace to build a topological quantum computer by tuning their quantum tunneling e ect. Using a designer Hamiltonian-the Kitaev toric-code model as an example, we study quantum tunneling e ects of the surface codes and obtain its e ective theory. Finally, we show how to do topological quantum computation including the initialization, the unitary transformation and the measurement.

In this paper, based on the formulation of O (3) non-linear model, we study two dimensional−flux Hubbard model at half-filling. A quantum non-magnetic insulator is explored near metalinsulator transition that may be a possible candidate of spin liquid state. Such quantum nonmagnetic insulator on square lattice is not induced by frustrations. Instead, it originates from quantum spin fluctuations with relatively small e ective spin-moments. In the strong coupling limit, our results of spin velocity and spin order parameter agree with results obtained from earlier calculations.

The properties of a class of topological quantum phase transition (TQPT) are analyzed based on a model proposed by Haldane. We study the effect of finite temperature on this phase transition. We have found that finite temperature would drive this TQPT to be a crossover, while it is stable against the weak short range interaction. When the interaction is strong enough, however, this TQPT is unstable and other states would emerge. Then we investigate the effect of the on-site energy in the original haldane model. The critical di erence between our TQPT and the topological phase transition in conventional quantum Hall system is discussed. Finally, we discuss the potential application of our analysis to a topological phase transition proposed in a realistic system.

A mutual-Chern-Simons Lagrangian is derived as a minimal field theory description of the phasestring model for doped antiferromagnets. Such an e ective Lagrangian is shown to retain the full symmetries of parity, time-reversal, and global SU (2) spin rotation, in contrast to conventional Chern-Simons theories where first two symmetries are usually broken. Two ordered phases, i.e., antiferromagnetic and superconducting states, are found at low temperatures as characterized by "dual" Meissner e ects and dual flux quantization conditions due to the mutual-Chern-Simons gauge structure. A "dual" confinement in charge/spin degrees of freedom occurs such that no true spincharge separation is present in these ordered phases, but the spin-charge separation/deconfinement serves as a driving force in the unconventional phase transitions of these ordered states to disordered states.

We study several di erent Z2 topological ordered states in frustrated spin systems. The e ective theories for those di erent Z2 topological orders all have the same form-a Z2 gauge theory which can also be written as a mutual U(1)×U(1) Chern-Simons theory. However, we find that the di erent Z2 topological orders are reflected in di erent projective realizations of lattice symmetry in the same e ective mutual Chern-Simons theory. This result is obtained by comparing the groundstate degeneracy, the ground-state quantum numbers, the gapless edge state, and the projective symmetry group of quasi-particles calculated from the slave-particle theory and from the e ective mutual Chern-Simons theories. Our study reveals intricate relations between topological order and symmetry.