Multiple phase estimation in quantum cloning machines

TL;DR

This paper analyzes the effectiveness of quantum cloning machines for multiple phase estimation using quantum Fisher information, demonstrating that phase-covariant cloning outperforms universal cloning in this context.

Contribution

It introduces a QFIM-based approach to evaluate quantum cloning machines for phase estimation, providing analytical formulas and comparing performance of UQCM and PQCM.

Findings

PQCM outperforms UQCM in QFIM measures

Analytical formulas for QFIM in d-dimensional states

Method applicable to various cloning schemes

Abstract

Since the initial discovery of the Wootters-Zurek no-cloning theorem, a wide variety of quantum cloning machines have been proposed aiming at imperfect but optimal cloning of quantum states within its own context. Remarkably, most previous studies have employed the Bures fidelity or the Hilbert-Schmidt norm as the figure of merit to characterize the quality of the corresponding cloning scenarios. However, in many situations, what we truly care about is the relevant information about certain parameters encoded in quantum states. In this work, we investigate the multiple phase estimation problem in the framework of quantum cloning machines, from the perspective of quantum Fisher information matrix (QFIM). Focusing on the generalized d-dimensional equatorial states, we obtain the analytical formulas of QFIM for both universal quantum cloning machine (UQCM) and phase-covariant quantum cloning machine (PQCM), and prove that PQCM indeed performs better than UQCM in terms of QFIM. We highlight that our method can be generalized to arbitrary cloning schemes where the fidelity between the single-copy input and output states is input-state independent. Furthermore, the attainability of the quantum Cramer-Rao bound is also explicitly discussed.