Quantum Monte Carlo(QMC) is one of the most powerful and successful methods to simulate many-body systems. Unfortunately, there is always a notorious problem called “Sign Problem”, especially in fermion systems or frustrated spin models. And the solution of the Sign Problem is considered to be nondeterministic polynomial (NP) hard. In this talk, I will briefly introduce the definition of Sign Problem, some explanations of its origin, and some ideas for trying to solve it[3,4,5]. I will try to avoid going into too much detail and hope to explain the ideas as much as possible.
 Matthias Troyer et al. Phys. Rev. Lett. 94, 170201(2005)
 Mauro Iazzi et al. Phys. Rev. B 93, 115102 (2016)
 Congjun Wu et al. Phys. Rev. B 71, 155115 (2005)
 Lei Wang et al. Phys. Rev. Lett. 115, 250601 (2015)
 Z. C. Wei et al. Phys. Rev. Lett. 116, 250601 (2016)