Objectivity, basis ambiguity, and the classical simulability of concordant quantum computation
Concordant quantum computations, in which circuits whose register carries zero quantum discord with respect to every subsystem at every step, contain neither entanglement nor any other form of quantum correlation, yet no efficient classical simulation of them is known beyond restricted gate sets. We argue that the obstruction is not a shortage of correlations but the absence of a uniquely identifiable local product basis, and we develop this \emph{basis ambiguity} on two fronts. Quantitatively, we introduce the Basis Ambiguity Measure $β\in[0,1]$, the normalized dimension of the manifold of admissible diagonalizing bases on a gate support, and prove a sharp equivalence: a concordance-preserving gate that defeats the Local Basis Finder of Cable and Browne exists if and only if $β>0$. Both directions are constructive. Physically, we identify the resource whose absence the obstruction is: \emph{basis objectivity}, a redundant and independently accessible record of the local basis. We show that an environment in spectrum broadcast form, together with the system--environment interaction map, replaces the NP-hard degeneracy test of Eastin by a fixed number of pairwise fragment comparisons per gate, at a cost independent of register size and circuit depth; the two ingredients live on different axes (state and dynamics), and neither alone suffices. We further prove that a theory in which every state has a strictly unique decomposition into perfectly distinguishable pure states has a simplex state space and lacks the continuity content of Hardy's fifth axiom, so that $β$ is a computational witness of that axiom. Finally, as a proof of principle that basis ambiguity carries operational content, we present a concordant encryption protocol with hidden local bases, together with an explicit account of what its security does and does not rest on.