Quantum algorithm for evaluating operator size with Bell measurements

Operator size growth describes the scrambling of operators in quantum dynamics and stands out as an essential physical concept for characterizing quantum chaos. Important as it is, a scheme for the direct measuring of operator size on a quantum computer is still absent. Here, we propose a quantum algorithm for the direct measuring of the operator size and its distribution based on Bell measurement. The algorithm is verified with spin chains and, meanwhile, the effects of Trotterization error and quantum noise are analyzed. It is revealed that saturation of operator size growth can be due to quantum chaos itself or be a consequence of quantum noise. Nevertheless, it is found that the error mitigation will effectively reduce the influence of noise, so as to restore the distinguishability of oscillation and saturation behaviors of operator scrambling. Our work provides a feasible protocol for investigating quantum chaos on noisy quantum computers by measuring operator size growth.