The Complexity of Approximating Complex-Valued Ising and Tutte Partition Functions

Monday, March 16, 2015 - 9:30am - 10:10am
Klaus 2443
Leslie Goldberg (University of Oxford)
We study the complexity of approximating the Ising and Tutte partition functions with complex parameters. Quantum computation is related to probabilistic computation and our results are motivated by the study of the quantum complexity classes BQP and IQP. Recent results show how to encode quantum computations as evaluations of classical partition functions. These results rely on interesting and deep results about quantum computation in order to obtain hardness results about the difficulty of (classically) evaluating the partition functions for certain fixed parameters. The motivation for our work is to study more comprehensively the complexity of (classically) approximating these partition functions with complex parameters. Partition functions are combinatorial in nature and quantifying their approximation complexity does not require a detailed understanding of quantum computation. Using combinatorial arguments, we give a full classification of the complexity of multiplicatively approximating the norm and additively approximating the argument of the Ising partition function for complex edge interactions. We also study the norm approximation problem in the presence of external fields, for which we give a complete dichotomy when the parameters are roots of unity. Previous results were known just for a few such points, and we strengthen these results from BQP-hardness to #P-hardness. Moreover, we show that computing the sign of the Tutte polynomial is #P-hard at certain points related to the simulation of BQP. Using our classifications, we then revisit the connections to quantum computation, drawing conclusions that are a little different from (and incomparable to) ones in the quantum literature, but along similar lines. No knowledge of quantum computation will be assumed in the talk.

(Joint work with Heng Guo)