  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.
