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We show how to approximately represent a quantum state using the square root of the usual amount of classical memory. The classical representation of an n-qubit state psi consists of its inner products with O(sqrt{2^n}) stabilizer states. A quantum state initially specified by its 2^n entries in the computational basis can be compressed to this form in time O(2^n poly(n)), and, subsequently, the compressed description can be used to additively approximate the expectation value of an arbitrary observable. Our compression scheme directly gives a new protocol for the vector in subspace problem with randomized one-way communication complexity that matches (up to polylogarithmic factors) the optimal upper bound, due to Raz. We obtain an exponential improvement over Raz’s protocol in terms of computational efficiency.
@InProceedings{gosset_et_al:LIPIcs.TQC.2019.8,
author = {Gosset, David and Smolin, John},
title = {{A Compressed Classical Description of Quantum States}},
booktitle = {14th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2019)},
pages = {8:1--8:9},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-112-2},
ISSN = {1868-8969},
year = {2019},
volume = {135},
editor = {van Dam, Wim and Man\v{c}inska, Laura},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.TQC.2019.8},
URN = {urn:nbn:de:0030-drops-104005},
doi = {10.4230/LIPIcs.TQC.2019.8},
annote = {Keywords: Quantum computation, Quantum communication complexity, Classical simulation}
}