Curator's Take
This article shows that truly memoryless (Markovian) noise is the exception rather than the rule in generic open‑quantum dynamics, with only a tiny finite slice of randomly generated two‑step qubit maps fitting a Markovian description. By quantifying how often different non‑Markovian witnesses and quantum‑memory criteria are triggered, it gives hardware engineers a concrete sense of how prevalent memory effects will be when modeling real devices, echoing recent pushes toward more realistic noise models for error mitigation and fault tolerance. The work also clarifies the relationships between various definitions of non‑Markovianity, helping the community align theory with the increasingly noisy, temporally correlated environments seen in today’s quantum processors.
— Mark Eatherly
Summary
A profound understanding of decoherence and dissipation in quantum dynamics is crucial for the realistic modeling of the evolution of quantum systems. In open quantum dynamics one distinguishes between a memoryless, so-called Markovian evolution and dynamics incorporating memory effects, termed non-Markovian. In this work we study how prevalent memory effects are in the set of all such dynamics. We thus investigate how often a Markovian description is applicable. This question is approached by investigating randomly generated two-step qubit dynamics with respect to different concepts and witnesses of non-Markovianity. We observe that almost all dynamics are non-Markovian, and only a small (yet finite) fraction is Markovian. Furthermore, we study how this proportion changes when considering certain subclasses such as lower rank or mixed-unitary dynamics. Importantly, our results shed light on the relative ratios of -- and interrelations between -- the sets of dynamics that are non-Markovian with respect to different criteria. Finally, we investigate the fraction of dynamics in which the memory effects are necessarily of quantum nature and establish a connection between two recently developed concepts that characterize the quantumness of memory in non-Markovian dynamics.