Tighter quantum uncertainty relations following from a general probabilistic bound

Uncertainty relations (URs) such as the Heisenberg-Robertson or the time-energy UR are often considered to be hallmarks of quantum theory. Here, a simple derivation of these URs is presented based on a single classical inequality from estimation theory, a Cramér-Rao-like bound. The Heisenberg-Robertson UR is then obtained by using the Born rule and the Schrödinger equation. This allows a clear separation of the probabilistic nature of quantum mechanics from the Hilbert space structure and the dynamical law. It also simplifies the interpretation of the bound. In addition, the Heisenberg-Robertson UR is tightened for mixed states by replacing one variance by the quantum Fisher information. Thermal states of Hamiltonians with evenly gapped energy levels are shown to saturate the tighter bound for natural choices of the operators. This example is further extended to Gaussian states of a harmonic oscillator. For many-qubit systems, we illustrate the interplay between entanglement and the structure of the operators that saturate the UR with spin-squeezed states and Dicke states.

Figure 1

Comparison of the left- (dashed) and right- (solid) hand sides in Eq. (13) for $\left|S\right(\mu )\rangle$ with $n=100$. The numbers next to solid lines indicate the $k$ used for a numerical search within the ansatz set (14) to maximize the right-hand side of Eq. (13). Choosing $k=1$ corresponds to the spin-squeezed inequalities. Clearly shown is the limited range of each $A_k$ for a tight bounding of the QFI. We remark that the presented curves are lower bounds on the actual optimal $A_k$.