,
Guangxu Yang
,
Penghui Yao
Creative Commons Attribution 4.0 International license
We investigates a model of hybrid classical-quantum communication complexity, in which two parties first exchange classical messages and subsequently communicate using quantum messages. We study the trade-off between the classical and quantum communication for composed functions of the form f∘Gⁿ, where f:{0,1}ⁿ → {±1} and G is an inner product function of Θ(log n) bits. To prove the trade-off, we establish a novel lifting theorem for hybrid communication complexity. This theorem unifies two previously separate lifting paradigms: the query-to-communication lifting framework for classical communication complexity and the approximate-degree-to-generalized-discrepancy lifting methods for quantum communication complexity. Our hybrid lifting theorem therefore offers a new framework for proving lower bounds in hybrid classical-quantum communication models.
As a corollary, we show that any hybrid protocol communicating c classical bits followed by q qubits to compute f∘Gⁿ must satisfy c+q² = Ω(max{deg(f), bs(f)}⋅log n), where deg(f) is the degree of f and {bs}(f) is the block sensitivity of f. For read-once formula f, this yields an almost tight trade-off: either they have to exchange Θ(n⋅log n) classical bits or Θ(√n ⋅ log n) qubits, showing that classical pre-processing cannot significantly reduce the quantum communication required. To the best of our knowledge, this is the first non-trivial trade-off between classical and quantum communication in hybrid two-way communication complexity.
@InProceedings{wu_et_al:LIPIcs.ICALP.2026.155,
author = {Wu, Xudong and Yang, Guangxu and Yao, Penghui},
title = {{A Lifting Theorem for Hybrid Classical-Quantum Communication Complexity}},
booktitle = {53rd International Colloquium on Automata, Languages, and Programming (ICALP 2026)},
pages = {155:1--155:24},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-428-4},
ISSN = {1868-8969},
year = {2026},
volume = {374},
editor = {Bhattacharya, Sayan and Nanongkai, Danupon and Benedikt, Michael and Puppis, Gabriele},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2026.155},
URN = {urn:nbn:de:0030-drops-265449},
doi = {10.4230/LIPIcs.ICALP.2026.155},
annote = {Keywords: Hybrid communication, Generalized discrepancy bound, Dual polynomial, Query-to-communication lifting}
}