We show that quantum states with "low stabilizer complexity" can be efficiently distinguished from Haar-random. Specifically, given an n-qubit pure state |ψ⟩, we give an efficient algorithm that distinguishes whether |ψ⟩ is (i) Haar-random or (ii) a state with stabilizer fidelity at least 1/k (i.e., has fidelity at least 1/k with some stabilizer state), promised that one of these is the case. With black-box access to |ψ⟩, our algorithm uses O(k^{12} log(1/δ)) copies of |ψ⟩ and O(n k^{12} log(1/δ)) time to succeed with probability at least 1-δ, and, with access to a state preparation unitary for |ψ⟩ (and its inverse), O(k³ log(1/δ)) queries and O(n k³ log(1/δ)) time suffice. As a corollary, we prove that ω(log(n)) T-gates are necessary for any Clifford+T circuit to prepare computationally pseudorandom quantum states, a first-of-its-kind lower bound.
@InProceedings{grewal_et_al:LIPIcs.ITCS.2023.64, author = {Grewal, Sabee and Iyer, Vishnu and Kretschmer, William and Liang, Daniel}, title = {{Low-Stabilizer-Complexity Quantum States Are Not Pseudorandom}}, booktitle = {14th Innovations in Theoretical Computer Science Conference (ITCS 2023)}, pages = {64:1--64:20}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-263-1}, ISSN = {1868-8969}, year = {2023}, volume = {251}, editor = {Tauman Kalai, Yael}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2023.64}, URN = {urn:nbn:de:0030-drops-175670}, doi = {10.4230/LIPIcs.ITCS.2023.64}, annote = {Keywords: Pseudorandom quantum states, Clifford + T, Haar random, Bell sampling, stabilizer formalism, stabilizer extent, stabilizer fidelity, learning theory, complexity theory} }
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