Novel approach for computing gradients of physical observables

We show that an infinitesimal step of gradient flow can be used for defining a novel approach for computing gradients of physical observables with respect to action parameters. Compared to the commonly used perturbative expansion, this approach does not require calculating any disconnected contribution or vacuum expectation value and can provide results up to three orders of magnitudes more precise. On the other hand, it requires a non-trivial condition to be satisfied by the flow action, the calculation of its force and its Laplacian, and the force of the observable, whose gradient needs to be measured. As a proof of concept, we measure gradients in $\beta$ of Wilson loops in a four-dimensional SU(3) Yang-Mills theory simulated on a $16^4$ lattice using the Wilson action.