In the shadow of the Hadamard test: Using the garbage state for good and further modifications

TL;DR

This paper proposes a hybrid quantum measurement approach combining the Hadamard test with classical shadows, enabling efficient extraction of multiple quantum state features on near-term devices.

Contribution

It introduces a novel method that integrates the Hadamard test with classical shadows, enhancing information extraction from quantum states beyond traditional techniques.

Findings

Effective estimation of eigenvalues using the Hadamard test

Access to fidelity, energy, and purity metrics via classical shadows

Potential for improved quantum state characterization on noisy devices

Abstract

The Hadamard test is naturally suited for the intermediate regime between the current era of noisy quantum devices and complete fault tolerance. Its applications use measurements of the auxiliary qubit to extract information, but disregard the system register completely. Separate advances in classical representations of quantum states via classical shadows allow the implementation of even global classical shadows with shallow circuits. This work combines the Hadamard test on a single auxiliary readout qubit with classical shadows on the remaining $n$-qubit work register. We argue that this combination inherits the best of both worlds and discuss statistical phase estimation as a vignette application. There, we can use the Hadamard test to estimate eigenvalues on the auxiliary qubit, while classical shadows on the remaining $n$ qubits provide access to additional features such as, (i) fidelity with certain pure quantum states, (ii) the initial state's energy and (iii) how pure and how close the initial state is to an eigenstate of the Hamiltonian. Finally, we also discuss how anti-controlled unitaries can further augment this framework.