Short-depth circuits for efficient expectation value estimation

The evaluation of expectation values $Tr\left[ρO\right]$ for some pure state $ρ$ and Hermitian operator $O$ is of central importance in a variety of quantum algorithms. Near optimal techniques developed in the past require a number of measurements $N$ approaching the Heisenberg limit $N=\mathcal{O}\left(1/ε\right)$ as a function of target accuracy $ε$. The use of Quantum Phase Estimation requires however long circuit depths $C=\mathcal{O}\left(1/ε\right)$ making their implementation difficult on near term noisy devices. The more direct strategy of Operator Averaging is usually preferred as it can be performed using $N=\mathcal{O}\left(1/ε^2\right)$ measurements and no additional gates besides those needed for the state preparation. In this work we use a simple but realistic model to describe the bound state of a neutron and a proton (the deuteron) and show that the latter strategy can require an overly large number of measurement in order to achieve a reasonably small relative target accuracy $ε_r$. We propose to overcome this problem using a single step of QPE and classical post-processing. This approach leads to a circuit depth $C=\mathcal{O}\left(ε^μ\right)$ (with $μ\geq0$) and to a number of measurements $N=\mathcal{O}\left(1/ε^{2+ν}\right)$ for $0<ν\leq1$. We provide detailed descriptions of two implementations of our strategy for $ν=1$ and $ν\approx0.5$ and derive appropriate conditions that a particular problem instance has to satisfy in order for our method to provide an advantage.

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