Out-of-Time-Ordered-Correlator Quasiprobabilities Robustly Witness Scrambling

TL;DR

This paper shows that the nonclassical negativity of quasiprobability distributions behind out-of-time-ordered correlators (OTOCs) is a more robust and sensitive indicator of quantum information scrambling in open systems than OTOCs alone.

Contribution

It introduces the nonclassical negativity of the quasiprobability distribution as a robust witness for scrambling, effective even in the presence of decoherence.

Findings

Nonclassical negativity evolves with timescales robust to decoherence.

QPD negativity is immune to false positives caused by decoherence.

Numerical simulations support the robustness of the proposed witness.

Abstract

Out-of-time-ordered correlators (OTOCs) have received considerable recent attention as qualitative witnesses of information scrambling in many-body quantum systems. Theoretical discussions of OTOCs typically focus on closed systems, raising the question of their suitability as scrambling witnesses in realistic open systems. We demonstrate empirically that the nonclassical negativity of the quasiprobability distribution (QPD) behind the OTOC is a more sensitive witness for scrambling than the OTOC itself. Nonclassical features of the QPD evolve with timescales that are robust with respect to decoherence and are immune to false positives caused by decoherence. To reach this conclusion, we numerically simulate spin-chain dynamics and three measurement protocols (the interferometric, quantum-clock, and weak-measurement schemes) for measuring OTOCs. We target experiments based on quantum-computing hardware such as superconducting qubits and trapped ions.