Quantum gradient evaluation through quantum non-demolition measurements

We discuss a quantum non-demolition measurement (QNDM) protocol to estimate the derivatives of cost function with computer. The function, which is supposed be classically hard evaluate, associated average value operator. Then computer used efficiently extract information about and its derivative by evolving system so-called variational circuit. To this aim, we propose use detector that allows us directly an observable, i.e., function. With respect standard direct approach, leads reduction number circuit iterations needed run circuits. advantage increases if want higher-order derivatives. also show presented approach can lead further in terms total logical gates These results make QNDM valuable alternative implementing