Even-parity precession protocol for detecting nonclassicality and entanglement

TL;DR

The paper presents an even-parity precession protocol capable of detecting various forms of nonclassicality and entanglement in quantum systems through single-variable measurements over time, including states previously undetectable by similar methods.

Contribution

It introduces a novel protocol that detects nonclassicality and entanglement without requiring multiple measurements, extending detection capabilities to states commuting with parity and GHZ entanglement in qubits.

Findings

Detects Wigner negativity in harmonic oscillators and spins.

Identifies non-Gaussian entanglement between oscillators.

Detects multipartite entanglement in spin ensembles.

Abstract

We introduce an even-parity precession protocol that can detect the nonclassicality of some quantum states using only measurements of a uniformly-precessing variable at different points in time. Depending on the system under study, the protocol may detect the Wigner negativity of a single quantum harmonic oscillator or of a single spin $j\geq 2$; the non-Gaussian entanglement of two harmonic oscillators; or genuine multipartite entanglement of a spin ensemble, whose total spin is an integer. Unlike in other nonclassicality tests, simultaneous or sequential measurements are not required. Our protocol can also detect states that commute with the parity operator, which were missed by similar protocols built from Tsirelson's original precession protocol. This work also closes a long-standing gap by showing the possibility of detecting the Greenberger-Horne-Zeilinger entanglement of an even number of qubits using only collective spin measurements.