Valiant's conjecture that VBP differs from VNP

Let $\mathrm{VBP}$ be the class of polynomial families computable by polynomial-size algebraic branching programs, and let $\mathrm{VNP}$ be Valiant's class of polynomially verifiable families. The inclusions satisfy $\mathrm{VBP}\subseteq\mathrm{VNP}$. Valiant's conjecture. We have

$$\mathrm{VBP}\neq\mathrm{VNP}.$$

This is a central separation question in algebraic complexity theory; the source gives no resolution.