Determination of All Unknown Pure Quantum States with Two Observables

TL;DR

This paper investigates the minimal measurement strategies for uniquely determining pure quantum states, demonstrating that two orthogonal bases can significantly narrow down candidates and that adaptive measurements can achieve unique state determination.

Contribution

It introduces a method to filter candidate states using two orthogonal bases and proposes an adaptive measurement scheme for unique pure state determination.

Findings

Two orthogonal bases can filter up to 2^{d-1} candidate states.

Adaptive measurement schemes can uniquely determine almost all pure qudits.

The approach avoids complex coefficients in measurement bases.

Abstract

Efficiently extracting information from pure quantum states using minimal observables on the main system is a longstanding and fundamental issue in quantum information theory. Despite the inability of probability distributions of position and momentum to uniquely specify a wavefunction, Peres conjectured a discrete version wherein two complementary observables, analogous to position and momentum and realized as projective measurements onto orthogonal bases, can determine all pure qudits up to a finite set of ambiguities. Subsequent findings revealed the impossibility of uniquely determining $d$-dimenisonal pure states even when neglecting a measure-zero set with any two orthogonal bases, and Peres's conjecture is also correct for $d=3$ but not for $d=4$. In this study, we show that two orthogonal bases are capable of effectively filtering up to $2^{d-1}$ finite candidates by disregarding a measure-zero set, without involving complex numbers in the bases' coefficients. Additionally, drawing inspiration from sequential measurements to directly calculate the target coefficients of the wavefunction using two complementary observables, we show that almost all pure qudits can be uniquely determined by adaptively incorporating a POVM in the middle, followed by measuring the complementary observable.