An almost convincing scheme for discriminating the preparation basis of quantum ensemble and why it will not work

TL;DR

The paper explores a measurement scheme aimed at identifying the preparation basis of quantum ensembles from a single copy, but finds that a biased random walk prevents its success, highlighting fundamental limitations.

Contribution

It proposes a measurement scheme to distinguish quantum preparation methods and analyzes why it fails due to a biased random walk effect.

Findings

The scheme cannot reliably identify preparation basis from a single copy.

Numerical simulations show the scheme's failure contradicts initial intuition.

A biased classical random walk explains the scheme's limitations.

Abstract

Mixed states of a quantum system, represented by density operators, can be decomposed as a statistical mixture of pure states in a number of ways where each decomposition can be viewed as a different preparation recipe. However the fact that the density matrix contains full information about the ensemble makes it impossible to estimate the preparation basis for the quantum system. Here we present a measurement scheme to (seemingly) improve the performance of unsharp measurements. We argue that in some situations this scheme is capable of providing statistics from a single copy of the quantum system, thus making it possible to perform state tomography from a single copy. One of the byproduct of the scheme is a way to distinguish between different preparation methods used to prepare the state of the quantum system. However, our numerical simulations disagree with our intuitive predictions. We show that a counter-intuitive property of a biased classical random walk is responsible for the proposed mechanism not working.