1. Quantum Orders in an exact soluble model (pdf)
   Xiao-Gang Wen
   Phys. Rev. Lett. 90, 016803 (2003). quant-ph/0205004
   Constructed an exact soluble spin-1/2 model on square lattice
   The ground states of the model can have different quantum orders at different couplings.
   The model has topological degenerate ground states and gapless edge excitations.
   
 
2. Quantum Order: a Quantum Entanglement of Many Particles (pdf)
   Xiao-Gang Wen
   Physics Letters A 300, 175 (2002). cond-mat/0110397
   Pointed out that patterns of quantum entanglement in many-qubit systems are characterized and classified by topological/quantum orders. A non-trivial topological/quantum order implies a non-local entanglement.
   The paper was rejected by PRL (referee's comments) ;-(
   
 
3. Mean Field Theory of Spin Liquid States with Finite Energy Gaps and Topological Orders (pdf)
   Xiao-Gang Wen
   Phys. Rev. B44, 2664 (1991).
   Found robust topological degeneracy in a spin liquid state with time-reversal and parity symmetries.
   The previous topological degeneracies are due to Chern-Simons terms while the present topological degeneracy is due to a Z_2 gauge theory.
   
 
4. Ground state degeneracy of the FQH states in presence of random potential and on high genus Riemann surfaces (pdf)
   Xiao-Gang Wen and Qian Niu
   Phys. Rev. B41, 9377 (1990)
   Pointed out the robust topological degeneracy in FQH states.
   The existence of topological degeneracy is a sign of topological order in FQH states.
   The robust topological degeneracy can be used in fault-tolerant quantum computation (Fault-tolerant quantum computation by anyons, Kitaev, 1997).
   
 
5. Topological Orders in Rigid States (pdf)
   Xiao-Gang Wen
   Int. J. Mod. Phys. B4, 239 (1990)
   Pointed out the robust topological degeneracy in a spin liquid state on 2D square lattice.
   The existence of topological degeneracy is a sign of topological order in the spin liquid.
   Studied non-Abelian Berry phases between the degenerate ground states produced by deforming the Hamiltonian.
   If we use degenerate ground states to realize qubits, then the non-Abelian Berry phases allows us to manipulate those qubits and to perform fault-tolerant quantum computation ( Holonomic Quantum Computation Zanardi and Rasetti, Quantum Computation by Adiabatic Evolution, Farhi, Goldstone, Gutmann, and Sipser, 2000).
   Proposed a conjecture: the non-Abelian Berry phases in the moduli space completely characterizes topological orders.
   
 

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