We study a variant of QMA where quantum proofs have no relative phase (i.e. non-negative amplitudes, up to a global phase). If only completeness is modified, this class is equal to QMA [Grilo et al., 2014]; but if both completeness and soundness are modified, the class (named QMA+ by Jeronimo and Wu [Jeronimo and Wu, 2023]) can be much more powerful. We show that QMA+ with some constant gap is equal to NEXP, yet QMA+ with some other constant gap is equal to QMA. One interpretation is that Merlin’s ability to "deceive" originates from relative phase at least as much as from entanglement, since QMA(2) ⊆ NEXP.
@InProceedings{bassirian_et_al:LIPIcs.ITCS.2024.9, author = {Bassirian, Roozbeh and Fefferman, Bill and Marwaha, Kunal}, title = {{Quantum Merlin-Arthur and Proofs Without Relative Phase}}, booktitle = {15th Innovations in Theoretical Computer Science Conference (ITCS 2024)}, pages = {9:1--9:19}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-309-6}, ISSN = {1868-8969}, year = {2024}, volume = {287}, editor = {Guruswami, Venkatesan}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2024.9}, URN = {urn:nbn:de:0030-drops-195370}, doi = {10.4230/LIPIcs.ITCS.2024.9}, annote = {Keywords: quantum complexity, QMA(2), PCPs} }
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