The Log Product Formula in quantum $K$-theory

We prove a formula expressing the $K$-theoretic log Gromov-Witten invariants of a product of log smooth varieties $V\times W$ in terms of the invariants of $V$ and $W$. The proof requires introducing log virtual fundamental classes in $K$-theory and verifying their various functorial properties. We introduce a log version of $K$-theory and prove the formula there as well.