Quantum Cramér–Rao bound

The Quantum Cramér–Rao bound (QCRB) is a fundamental result in quantum estimation theory. It generalizes the classical Cramér-Rao bound to the realm of quantum mechanics, providing a theoretical lower limit on the variance of unbiased estimators for quantum parameters. ### Key Concepts: 1. **Parameter Estimation**: In quantum mechanics, one often wishes to estimate parameters (like phase, frequency, etc.) of quantum states.