Curator's Take
AI Commentary
This article demonstrates that even in a quantum setting there can exist an “oracle world”—dubbed Quantum Pessiland—in which average‑case hardness of UP∩coUP holds yet the foundational primitives needed for virtually all quantum cryptographic protocols (EFI pairs or one‑way puzzles) are provably absent. By constructing both quantum and classical oracles that also collapse SampBQP to SampBPP, the work sharpens our understanding of where quantum advantage truly begins and highlights concrete complexity barriers to secure quantum encryption beyond standard assumptions. The result serves as a cautionary benchmark for researchers: without careful hardness foundations, many hoped‑for quantum cryptographic schemes may be impossible, underscoring the need to identify robust, oracle‑independent primitives before building practical protocols.
— Mark Eatherly
Summary
Pessiland is a world where NP is hard on average but one-way functions (OWFs) do not exist [Impagliazzo 1995]. Because almost all classical cryptographic primitives imply OWFs [Impagliazzo and Luby 1989], there is almost no classical cryptography in Pessiland. On the other hand, quantum cryptography can exist even when OWFs do not [Kretschmer 2021; Morimae and Yamakawa 2022; Ananth, Qian and Yuen 2022]. Is there a quantum analogue of Pessiland where NP is hard on average but even quantum cryptography does not exist? In this paper, we show that such a miserable world, Quantum Pessiland, exists: there is a quantum oracle relative to which $UP\cap coUP$ is hard on average against quantum polynomial-time algorithms with quantum advice, yet auxiliary-input EFI pairs do not exist. We also show that there is a classical oracle relative to which $UP\cap coUP$ is hard on average against quantum polynomial-time algorithms with quantum advice, yet classically-secure auxiliary-input one-way puzzles (OWPuzzs) do not exist. Almost all quantum cryptographic primitives imply EFI pairs or OWPuzzs, and therefore these results mean that there is almost no quantum cryptography relative to these oracles. We further show that relative to the classical oracle, SampBQP = SampBPP, and therefore there is no sampling-based quantum advantage in Quantum Pessiland. Finally, because our average-case hardness of $UP\cap coUP$ implies $P^{\#P}\not\subseteq i.o.BQP/qpoly$, our result also implies that a non-relativizing proof technique is necessary to construct OWPuzzs solely from $P^{\#P}\not\subseteq i.o.BQP/qpoly$, which gives a partial negative answer to the open problem of [Khurana and Tomer 2025].