Phase transition in Stabilizer Entropy and efficient purity estimation

TL;DR

This paper investigates a phase transition in Stabilizer Entropy related to non-Clifford resources, revealing a new efficient method for purity estimation that outperforms existing algorithms for certain quantum states.

Contribution

It uncovers a phase transition in Stabilizer Entropy and introduces a polynomial-resource protocol for purity estimation with exponential speed-up for highly entangled states.

Findings

Identifies a phase transition in residual subsystem Stabilizer Entropy.

Develops a polynomial-query purity estimation protocol.

Achieves exponential speed-up over current algorithms for certain states.

Abstract

Stabilizer Entropy (SE) quantifies the spread of a state in the basis of Pauli operators. It is a computationally tractable measure of non-stabilizerness and thus a useful resource for quantum computation. SE can be moved around a quantum system, effectively purifying a subsystem from its complex features. We show that there is a phase transition in the residual subsystem SE as a function of the density of non-Clifford resources. This phase transition has important operational consequences: it marks the onset of a subsystem purity estimation protocol that requires $poly(n)exp(t)$ many queries to a circuit containing $t$ non-Clifford gates that prepares the state from a stabilizer state. Then, for $t=O(\log_2 n)$, it estimates the purity with polynomial resources and, for highly entangled states, attains an exponential speed-up over the known state-of-the-art algorithms.