  Envariance, or environment-assisted invariance, is a recently identified
symmetry for maximally entangled states in quantum theory with important
ramifications for quantum measurement, specifically for understanding Born's
rule. We benchmark the degree to which nature respects this symmetry by using
entangled photon pairs. Our results show quantum states can be 99.66(4)%
envariant as measured using the quantum fidelity, and 99.963(5)% as measured
using a modified Bhattacharya Coefficient, as compared with a perfectly
envariant system which would be 100% in either measure. The deviations can be
understood by the less-than-maximal entanglement in our photon pairs.
