The Power of Unentangled Quantum Proofs with Non-negative Amplitudes
Fernando Granha Jeronimo, Pei Wu
TL;DR
This work analyzes unentangled quantum proofs with non-negative amplitudes, introducing and characterizing the classes $ extup{QMA}^+(2)$ and $ extup{QMA}^+_{ ext{log}}(2)$ and proving that NP problems admit constant-gap QMA$^+_{ ext{log}}(2)$ protocols for increasingly hard tasks. The authors design global, coherent verification protocols for small-set expansion (SSE), unique games (UG), and a PCP-based NEXP verifier, exploiting non-negative amplitudes and a suite of property-testing primitives to achieve constant completeness-soundness gaps with logarithmic proof sizes. A central technical advance is proving a robust analytic SSE property via sparse-support analysis and harnessing it to obtain NP $oldsymbol{ o}$ QMA$^+_{ ext{log}}(2)$ and eventually $ ext{NEXP}= ext{QMA}^+(2)$ through explicit, doubly explicit PCP constructions. The results provide a structural, complexity-theoretic understanding of unentangled quantum proofs and illuminate how non-entanglement can yield strong, scalable verification, with potential implications for resolving the long-standing QMA(2) versus NEXP question through gap amplification in the non-negative-amplitude regime.
Abstract
Quantum entanglement is a fundamental property of quantum mechanics and plays a crucial role in quantum computation and information. We study entanglement via the lens of computational complexity by considering quantum generalizations of the class NP with multiple unentangled quantum proofs, the so-called QMA(2) and its variants. The complexity of QMA(2) is a longstanding open problem, and only the trivial bounds QMA $\subseteq$ QMA(2) $\subseteq$ NEXP are known. In this work, we study the power of unentangled quantum proofs with non-negative amplitudes, a class which we denote $\text{QMA}^+(2)$. In this setting, we are able to design proof verification protocols for problems both using logarithmic size quantum proofs and having a constant probability gap in distinguishing yes from no instances. In particular, we design global protocols for small set expansion, unique games, and PCP verification. As a consequence, we obtain NP $\subseteq \text{QMA}^+_{\log}(2)$ with a constant gap. By virtue of the new constant gap, we are able to ``scale up'' this result to $\text{QMA}^+(2)$, obtaining the full characterization $\text{QMA}^+(2)$=NEXP by establishing stronger explicitness properties of the PCP for NEXP. One key novelty of these protocols is the manipulation of quantum proofs in a global and coherent way yielding constant gaps. Previous protocols (only available for general amplitudes) are either local having vanishingly small gaps or treat the quantum proofs as classical probability distributions requiring polynomially many proofs thereby not implying non-trivial bounds on QMA(2). Finally, we show that QMA(2) is equal to $\text{QMA}^+(2)$ provided the gap of the latter is a sufficiently large constant. In particular, if $\text{QMA}^+(2)$ admits gap amplification, then QMA(2)=NEXP.