The Trade-off in Quantum Metrology: Excessive Precision Compromise Accuracy even with Unlimited Resources

Abstract: Precision and accuracy, as two crucial criteria for quantum metrology, have previously lacked rigorous definitions and distinctions. In this paper, we provide a unified definition of precision and accuracy from the perspective of distinguishing neighboring quantum states. Using the quantum Cram\'er-Rao bound as a lower bound for precision, we find that the corresponding accuracy will fall short of expectations, because the bias of the parameter estimation cannot be ignored. Given that probability estimation is unbiased, defining precision from the perspective of probability distributions provides a more comprehensive approach. This leads to a correction of the traditional precision lower bound by a factor of 2. The trade-off between precision and accuracy shows that precision can be further improved by sacrificing accuracy, while it should be restricted by inherent precision limit. The inherent precision limit, determined by the number of sampling, can reach the Heisenberg scaling even without entanglement resources, which, however, comes at the cost of significantly reduced accuracy. We show that accuracy may actually decrease with increasing sampling when one pursues excessive precision, which indicates the trade-off should be considered even with unlimited resources.