Short-depth circuits for efficient expectation value estimation

Abstract

The evaluation of expectation values $Tr\left[\rho O\right]$ for some pure state $\rho$ 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 $N=\mathcal{O}\left(1/\epsilon \right)$ as a function of target accuracy $\epsilon$. The use of Quantum Phase Estimation requires however long circuit depths $C=\mathcal{O}\left(1/\epsilon \right)$ 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 $N=\mathcal{O}\left(1/{\epsilon }^2\right)$ 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 ${\epsilon }_r$. We propose to overcome this problem using a single step of QPE and classical post-processing. This approach leads to a circuit depth $C=\mathcal{O}\left({\epsilon }^{\mu }\right)$ (with $\mu \ge 0$) and to a number of measurements $N=\mathcal{O}\left(1/{\epsilon }^{2+\nu }\right)$ for $0<\nu \le 1$. We provide detailed descriptions of two implementations of our strategy for $\nu =1$ and $\nu \approx 0.5$ 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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