We show that for any fixed (2+1)-dimensional TQFT over ℂ of either Turaev-Viro-Barrett-Westbury or Reshetikhin-Turaev type, the problem of (exactly) computing its invariants on closed 3-manifolds is either solvable in polynomial time, or else it is #𝖯-hard to (exactly) contract certain tensors that are built from the TQFT’s fusion category. Our proof is an application of a dichotomy result of Cai and Chen [J. ACM, 2017] concerning weighted constraint satisfaction problems over ℂ. We leave for future work the issue of reinterpreting the conditions of Cai and Chen that distinguish between the two cases (i.e. #𝖯-hard tensor contractions vs. polynomial time invariants) in terms of fusion categories. We expect that with more effort, our reduction can be improved so that one gets a dichotomy directly for TQFTs' invariants of 3-manifolds rather than more general tensors built from TQFTs' fusion categories.
@InProceedings{bridges_et_al:LIPIcs.TQC.2025.5, author = {Bridges, Nicolas and Samperton, Eric}, title = {{Towards a Complexity-Theoretic Dichotomy for TQFT Invariants}}, booktitle = {20th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2025)}, pages = {5:1--5:21}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-392-8}, ISSN = {1868-8969}, year = {2025}, volume = {350}, editor = {Fefferman, Bill}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.TQC.2025.5}, URN = {urn:nbn:de:0030-drops-240548}, doi = {10.4230/LIPIcs.TQC.2025.5}, annote = {Keywords: Complexity, topological quantum field theory, dichotomy theorems, constraint satisfaction problems, tensor categories} }
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